Notes on As-Siyam (the fasting).
In a narration by Muslim, the Messenger of Allah (ﷺ) said, “The reward of every (good) deed of a person is multiplied from ten to seven hundred times. Allah (SWT) says: ‘The reward of observing Saum is different from the reward of other good deeds; Saum is for Me, and I Alone will give its reward.