Anas said, "I heard the Messenger of Allah, may Allah bless him and grant him peace, say, "Allah Almighty says, "O son of Adam! I will forgive you as long as you call on Me and have hope in Me, no matter what you do. Son of Adam, I do not care if your wrong actions reach to the clouds of heaven and then you ask Me for forgiveness, I will forgive you. Son of Adam, if you were to come with sins equivalent in weight to the whole earth and then meet Me having not associated anything with Me, I would come to you with the same amount of forgiveness."
Riyadh-as-Saliheen by Imam Al-Nawawi, volume 3, hadith number 442 - Sunni Hadith