Hadeeth Qudsi 34:

On the authority of Anas (mayAllah be pleased with him), who said: I heard the Messenger of Allah say: Allah the Almighty said:

O son of Adam, so long as you callupon Me and ask of Me, I shall forgive you for what you have done, andI shall not mind. O son of Adam, were your sins to reach the clouds ofthe sky and were you then to ask forgiveness of Me, I would forgiveyou. O son of Adam, were you to come to Me with sins nearly as great asthe earth and were you then to face Me, ascribing no partner to Me, Iwould bring you forgiveness nearly as great at it.

It was related by at-Tirmidhi(also by Ahmad ibn Hanbal). Its chain of authorities is sound.