I heard the Messenger of Allah (peace be upon him) say : “Allah the Almighty said : O son of Adam, so long as you call upon Me and ask of 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 of the sky and were you then to ask forgiveness of Me, I would forgive you. O son of Adam, were you to come to Me with sins nearly as great as the earth and were you then to face Me, ascribing no partner to Me, I would 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.