Decision that Penitence Purifies a Sinner

It has been described by Saduq (r.a.) on the authority of Saad Bin Tarif who has carried it from Asbagh
A man came to Hazrat Ali (a.s.) and submitted to him that he had committed a major crime and asked him to purify him. Hazrat Ali (a.s.) asked him whether he was suffering from any major disease. 
When he replied in the affirmative, Hazrat Ali (a.s.) asked him to go away, get the disease treated and then come back to him. 
When he came back Hazrat Ali (a.s.) asked him whether he repented over what he had done. 
The man said, “Yes” and wept bitterly. 
Then Hazrat Ali (a.s.) told him that repentance had purified him as sincere repentance and penitence was sufficient for purification. 

But it has been unanimously accepted that nobody else than an Imam can grant forgiveness like this.

Abu Turab, vol. 2, p. 118