Musnad Ahmad 30:3409

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: Allah the Almighty and Majestic said: "Every deed of the son of Adam is for him, for a good deed is [rewarded by] ten times its like, up to seven hundred times over, except for fasting; it is for Me, and I reward for it. He leaves his food for his desire for My sake, and he leaves his drink for his desire for My sake. It is for Me, and I reward for it."