Hadith #42 of the 40 Hadeeth Imam Nawawi:
What a very beautiful hadith!
From Anas, radiyallahu 'anhu, who said: I heard the Messenger of Allah, sallallahu 'alayhi wasallam, say:
"Allah the Almighty has said: 'O son of Adam, so long as you call upon Me, and hope in Me, I shall forgive you for what you have done, and I shall not mind. O son of Adam, were your sins to reach the clouds in the sky and were you then to ask forgiveness of Me, I shall forgive you. O son of Adam, were you to come to Me with an earthful of sins and were you then to face Me, without having associated anything with Me, I shall grant you an earthful of pardon.'"
[Recorded by Al-Tirmidhi, who said that it is a good and sound hadith]