Musnad Ahmad 26:706

It was read to my father: Amr ibn Mujamma' Abu al-Mundhir al-Kindi narrated to you, he said: Ibrahim al-Hajari informed us, from Abu al-Ahwas, from Abdullah ibn Mas'ud, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, Allah, the Almighty and Majestic, has made the good deed of the son of Adam ten times its like, up to seven hundred times, except for fasting. Fasting is for Me, and I reward for it. The fasting person has two joys: a joy at the time of his breaking the fast, and a joy on the Day of Resurrection. And the odor of the mouth of the fasting person is more pleasant to Allah than the scent of musk."