Abu Dawud Book 2, Hadith Number 1516.

Chapter : On seeking pardon and repentance.

Narated By Abu Bakr as-Siddiq : Asma’ bint al-Hakam said: I heard Ali say: I was a man; when I heard a tradition from the Apostle of Allah (PBUH), Allah benefited me with it as much as He willed. But when some one of his companions narrated a tradition to me I adjured him. When he took an oath, I testified him.

Abu Bakr narrated to me a tradition, and AbuBakr narrated truthfully. He said: I heard the apostle of Allah (PBUH) saying: When a servant (of Allah) commits a sin, and he performs ablution well, and then stands and prays two rak’ahs, and asks pardon of Allah, Allah pardons him. He then recited this verse: “And those who, when they commit indecency or wrong their souls, remember Allah” (iii.134).