Musnad Ahmad 30:3562

Rawh narrated to us, Malik narrated to us, from Abu al-Zinad, from al-A’raj, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "By the One in Whose hand is my soul, the change in the breath of the fasting person is more pleasant to Allah than the scent of musk. He leaves his food, his drink, and his desire for My sake. So, the fasting is for Me, and I reward for it. Every good deed is [rewarded] by ten times its like, up to seven hundred times, except for fasting, for it is for Me, and I reward for it."