On the authority of Anas (may Allah be pleased with him) who said:
I heard the Messenger of Allah (peace and blessings of Allah be upon him) say, "Allah the Almighty has said: 'O Son of Adam, as long as you invoke 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 you then asked forgiveness from 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 as it [too].' " It was related by at-Tirmidhi, who said that it was a hasan hadeeth.
Reference: 40 Hadith Nawawi 42
English translation: Hadith 42