Ishaq bin Ayyub al-Wasiti (may Allah have mercy on him) narrates from his father that he heard a man saying to Sufyan bin Uyaynah (may Allah have mercy on him): O Abu Muhammad! Tell us about what the Prophet (peace and blessings be upon him) narrates from his Lord: "Every deed of the son of Adam is for him except fasting, for it is for Me and I alone will give its reward." Ibn Uyaynah (may Allah have mercy on him) said: This is among the finest of hadiths and is firmly established. When the Day of Resurrection comes, Allah Almighty will take account of His servant and will settle whatever wrongs he owes out of all his deeds, until nothing remains but fasting; then Allah Almighty will remove the remaining wrongs and admit him into Paradise because of the fasting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8510