Mu'awiya ibn Abu Ishaq quoting Sa'eed ibn Jubayr saying,
I asked Ibn Abbas once about the reward of one who fasts during the month of Ramadan knowing its greatness. He said: 'O Ibn Jubayr! Get ready to listen to what your ears have never heard before, nor your heart has ever experienced, nor your soul has ever reckoned regarding that about which you have inquired! What you are seeking is the knowledge of the first generations and the last!
So I left him and prepared myself to meet him again. I returned to him at early daybreak. Having said the fajr prayers (together), I reminded him of the tradition which I had sought, so he turned his face to me and said,
Listen carefully to what I am going to tell you. I have heard the Messenger of Allah (pbuh) saying: 'Had you ever come to know about your rewards during the month of Ramadan, you would surely have thanked the Almighty a great deal more (than you usually do).
When the first night is over, Allah, the Almighty and the Exalted One, forgives the sins committed by all members of my nation, the ones committed in secrecy and the ones committed in public, and He elevates your status two thousand degrees and builds you fifty towns in Paradise.
On the next day, He rewards you for every step you take during that day with the rewards of one who adored Him for a full year and the reward of one of His prophets, and He will reward you as though you had performed the fast for a full year.
On the third day, the Exalted and Dear One grants you a dome in Paradise for each hair on your body, a dome of a white pearl on top of which are twelve thousand light houses and at the bottom of which are twelve thousand houses in each one of which there are one thousand beds and on each bed of which there is a nymph (huri) with large lovely eyes, each served by one thousand servants the head-covering of each one of them is better than this world and everything in it.
On fifth day, He builds you in Paradise a million cities in each one of which there are seventy thousand houses, inside each one of which there are seventy thousand tables, and on each table there are seventy thousand bowls, and in each bowl there are sixty thousand types of food each one of which is different from the other.
On the sixth day, He will grant you in the Abode of Peace a hundred thousand towns in each one of which there are a hundred thousand rooms, in each room there are a hundred thousand beds of gold the length of each is a thousand yards, and on each bed is a hun wife with large lovely eyes whose hair has thirty thousand locks braided with pearls and sapphires, and each lock is carried by a hundred concubines.
On the seventh day, the Almighty grants you in the Garden of Bliss the rewards of forty thousand martyrs and forty thousand siddeeqs.
On the eighth day, Allah Almighty grants you the rewards of the good deeds of sixty thousand worshippers and sixty thousand ascetics.
On the ninth day, Allah, the Exalted One, gives you what is equal to what He gives a thousand scholars and a thousand devotees and a thousand warriors fighting for Allah in a foreign land.
On the tenth day, He gives you the fulfillment of seventy thousand of your worldly wishes and orders the sun, the moon, the stars, the animals, the birds, the beasts, every rock and every rain-drop, everything wet and everything dry, all fish in the oceans and all leaves on the trees, to pray for your forgiveness.
On the eleventh day, the Exalted and Mighty One grants you the rewards whereby He rewards one who performs the pilgrimage and 'umra four times and one who performs the pilgrimage with His prophets and the 'umra with every siddeeq or martyr.
On the twelfth day, He takes upon Himself to replace your sins with good deeds, then He multiplies your good deeds many times and gives you the rewards of each of your good deeds a million times.
On the thirteenth day, Allah Almighty grants you what He grants the devotees of Mecca and Medina and bestows upon you an intercession for each and every stone and rain drop between Mecca and Medina.
On the fourteenth day, He treats you as though you had met and followed in the footsteps of Adam, Noah, Abraham, Moses, David and Solomon [peace be upon all of them], and as though you had worshipped the Almighty Allah in the company of His prophets for two hundred years.
On the fifteenth day, He fulfills ten of your worldly wishes and those of the hereafter and grants you what He granted Job (as), then He orders the angels who bear the 'Arsh to pray for your forgiveness and grants you on the Day of Resurrection forty lights: ten on your right, ten on your left, ten before you and ten behind you.
On the sixteenth day, the Almighty grants you sixty outfits to wear as soon as you abandon your grave and a she-camel to ride, and He will send a cloud to overshadow you to protect you from the heat of that Day.
On the seventeenth day, the Almighty Allah says: 'I have forgiven them and their parents and exempted them from having to undergo the hardships of the Day of Resurrection.'
On the eighteenth day, the Praised and Exalted One orders Gabriel, Michael and Israfil as well as the angels who bear the 'Arsh and all archangels to seek forgiveness for the nation of Muhammad (pbuh) till the next year, and He will also grant you on the Day of Resurrection whatever rewards He grants to those who participated in the Battle of Badr].
On the nineteenth day, all angels in the heavens and on earth will have already sought permission of their Lord to visit your graves and to bring you every day a present and a drink [as long as you remain in the barzakh].
So, if you complete your fast for twenty full days, the Almighty Allah sends you seventy thousand angels to protect you from every accursed devil, and He will have granted you for each day of your fast your rewards as though you fasted a hundred years, and He will set a ditch between you and hell and grant you the rewards of all those who recited the Torah, the Gospel, the Psalms and the Holy Qur'an, and will write for you for each feather on Gabriel the reward of a full year and will grant you the rewards of those who glorify Him at the 'Arsh and Kursi and will marry you to a thousand nymphs for each of the verses of the Qur'an.
On the twenty-first day, the Almighty expands your grave a thousand parasangs and lifts the darkness and loneliness of your graves and makes your graves look like the graves of the martyrs and your faces like the face of Joseph son of Jacob (as).
On the twenty second day, the Almighty dispatches the angel of death as He dispatches him to His prophets to remove your worldly worries and the torment of the hereafter.
On the twenty third day, you will pass on the Straight Path in the company of the prophets, the first to follow the prophets, and the martyrs, as if you had fed each orphan and clothed everyone who needed to be clothed. On the twenty-fourth day, you will not leave this life before each one of you sees the place reserved for him or her in Paradise and is given the rewards of a thousand sick and a thousand who go back to their creed and will grant you the rewards of one who freed a thousand captives from the descendants of Ishmael (as).
On the twenty-fifth day, Allah will have built you under His 'Arsh a thousand green domes on top of each one of which is a tent of light. The Almighty and Exalted One will then say: 'O followers of Muhammad! I am your Lord and you are My servants! Enjoy the shade of My 'Arsh in these domes and eat and drink with enjoyment, for there will be no fear on you, nor will you grieve. O nation of Muhammad! By My Dignity and Greatness! I shall dispatch you to Paradise in a way which will amaze the first generations and the last, and I shall crown each one of you with a thousand light crowns, and I shall provide for each one of you a she camel whose reins are made of light, and in it are a thousand gold rings, in each is an angel looking after it, in the hand of each angel is a light rod so that he may enter Paradise without a reckoning.'
And on the twenty sixth day, Allah will look at you with compassion and will forgive all your sins except those of shedding innocent blood or robbing people's wealth, and He will grant you every day a thousand barriers against backbiting, lying and slandering.
On the twenty-seventh day, He will consider you as though you had aided every believing man and woman and clothed seventy thousand naked persons and equipped a thousand soldiers to camp in a foreign land to defend Islam, and as if you have recited every book Allah has revealed to His prophets.
On the twenty-eighth day, Allah will have built you in Paradise a hundred thousand light cities and granted you in the garden of bliss a hundred thousand silver mansions and a hundred thousand cities in each one of which there are a thousand rooms, and granted you in the garden of greatness a hundred thousand pulpits of musk inside each one of which there is a thousand saffron houses in each one of which there are a thousand beds of pearls and sapphires and on each bed a wife of the huns with large lovely eyes.
So if you complete your fast till the twenty ninth day, the Almighty Allah will grant you a million quarters, inside each quarter is a white dome underneath which is a white camphor bed on which there are a thousand mattresses of green silk on each one of which there is a hun decorated with seventy thousand ornaments and crowned with eighty thousand locks each one of which is decorated with diamonds and sapphires.
So if you finish thirty complete days of fast, the Almighty will have granted you for each day the rewards of a thousand martyrs and a thousand foremost believers in His Prophets, and He will have assigned for you the rewards of fifty years of adoration, and He will have decreed a clearance for you from hell and a passage on the Straight Path and a security against the torment. One of the gates of Paradise is called al-Rayyan, and it shall never be opened before the Day of Resurrection. It will be opened for those among the nation of Muhammad (pbuh) who performed the fast. Ridwan, custodian of Paradise, will call out saying: 'O followers of Muhammad! Come to the al-Rayyan gate!' So he will let my nation enter Paradise through that gate. Therefore, if one is not forgiven during the month of Ramadan, in which month can he be forgiven? There is neither will nor strength except from Allah; Allah suffices us, and what a great Helper He is!
This lengthy tradition is also recorded on pp. 183-185, Vol. 8, of Bihar al-Anwar.
(Thanks to Mr. Firoz Meghani for sharing this beautiful tradition)