Prophet Muhammad said: "Fasting is a protection. So, a person observing fasting should avoid sexual activity, and should not behave foolishly or impudently. If somebody fights with him or abuses him, he should tell him twice, 'I am fasting.' By the One in whose hand is the soul of Muhammad, the smell that comes from the mouth of a fasting person is better before Allah than the fragrance of musk. (Allah says about the fasting person), 'He left his food, drink and desires for My sake. Fasting is for Me, and I will reward (the fasting person) for it.'” [Bukhari]
The reward for good deeds is multiplied 10 to 700 times – except for fasting. The reward for fasting is not limited to 700 times. Allah will choose to reward for fasting based on His generosity and great bounty – only He knows how much the reward will be. The reason for this is twofold:
One: Every other act of worship is visible to others, whereas fasting is a secret between the servant and his Lord. No one is following a fasting person all day to observe his actions, so only Allah and the fasting person know the truth – that it is done for His sake alone.
Two: Other acts of worship include spending money (e.g., charity) or using one's body (e.g., prayer), whereas fasting only requires that one stay away from desires for a period of time. This helps tame one's desires, teaches him patience & perseverance, and allows him to reflect deeply.