Musnad Ahmad 30:2586

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Abu Salih, from Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Every deed of the son of Adam is multiplied; a good deed is ten times its like, up to seven hundred times, up to what Allah wills. Allah the Almighty and Majestic says: 'Except for fasting, for it is Mine, and I reward for it. He leaves his food and his desire for My sake.'" "For the fasting person are two joys: a joy at the time of his breaking fast, and a joy at the time of meeting his Lord." "And the change in the breath of the mouth of the fasting person is better to Allah than the scent of musk. Fasting is a shield, fasting is a shield."