Sahih Bukhari Volume 6, Book 60, Hadith Number 6.

Narrated By Abu Huraira : The Prophet said, “It was said to the children of Israel, ‘Enter the gate (of the town), prostrate (in humility) and say: Hittatun (i.e. repentance) i.e. O Allah! Forgive our sins.’ But they entered by dragging themselves on their buttocks, so they did something different (from what they had been ordered to do) and said, ‘Hittatun,’ but added, “A grain in a hair.”