Sahih Muslim Book 1, Hadith Number 238.

Chapter : Whenever a person intends to do a good deed, it is recorded, but whenever he intends to commit evil, it is not written.

This hadith has been narrated with another chain of transmitters with the addition of these words: Allah would even wipe out (the evil committed by a man) and Allah does not put to destruction anyone except he who is doomed to destruction.