Musnad Ahmad 30:2869

Ishaq narrated to us, Malik informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that 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 (the Almighty and Majestic) says: 'He only abandons his desire, his food, and his drink 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.'"