The Holy Prophet (pbuh) said: “The sigh of the people of Hell is mostly because of the postponement of repenting.”
Imam Baqir (A.S.) said: “He who repents of his sins is as the one who has not any (burden of) sin.”
Imam Amir-ul-Mumineen Ali (A.S.) said: “How numerous procrastinators there are who postpone (repenting and doing good deeds) until when death overtakes them!”
The Holy Prophet (pbuh) told Ali-ibn-Abitalib (A.S.): “O Ali! blessed is he who Allah looks upon while he is weeping for the sin that none is aware of except Allah.”
(Various traditions emphasize that one must not let others know of his sins. One must make confessions about his sins only to Allah, and, then, repent)
Reference: A Bundle of Flowers (Traditions of the Ahlul-Bayt) pg.84-85