Musnad Ahmad 30:376

He said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Allah, the Mighty and Majestic, says: 'Every deed of the son of Adam is for him, except for fasting; for it is Mine, and I reward for it. He leaves his food and his drink for My sake, so his fasting is for Me, and I reward for it.' Every good deed is [multiplied by] ten times its like, up to seven hundred times, except for fasting; for it is Mine, and I reward for it."