The Holy Prophet (S) said: "The sigh of the people of Hell is mostly because of the postponement of repenting."
Imam al-Baqir (‘a) said: "He who repents of his sins is as the one who has not any (burden of) sin.
Wasa'il-ush-Shi'ah vol. 16, p. 74
Imam Amir ul Mu'mineen ‘Ali (‘a) said: "How numerous procrastinators there are who postpone (repenting and doing good deeds) until when death overtakes them!
Ghurar-ul-Hikam, p. 240
The Holy Prophet (S) told ‘Ali-ibn-Abi-Talib (‘a): "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.)
Bihar-ul-Anwar, vol. 77, p. 63
The Holy Prophet (S) said: "During the third part of every night and on the night before Friday from the beginning (until the break of dawn) Allah sends an angel unto the sky of the earth in order to call: 'Is there any needy who asks that I may bestow upon him his need? Is there any repentant one that I may return to him? Is there any seeker of forgiveness that I may forgive him?'"
Islamic traditions teach that sleeping on the night before Friday is called 'the regret sleep'; for, on the Reckoning Day, people will regret very much that they had been asleep on the nights before Fridays.
Bihar-ul-Anwar, vol. 3, p. 314
Imam as-Sadiq, the sixth Imam, (‘a) said: "Shut the doors of sins through seeking refuge to God, and open the doors of obedience by (reciting) 'Bismillah'."1
Bihar-ul-Anwar, vol. 92, p. 216