Musnad Ahmad 4228

It was narrated that 'Abdullah bin Mas'ood said: The Messenger of Allah (ﷺ) said: Verily, Allah has made the reward for the good deed of the son of Adam ten like it, up to seven hundred fold, except fasting, for fasting is for Me and I and the One Who will reward for it. The fasting person has two moments of joy: joy when he breaks his fast and joy on the Day of Resurrection. And the smell from the mouth of the fasting person is better before Allah than the fragrance of musk.