Fasting generally means “to abstain from something.” For example, a verse in the Qur’an reads: “I have vowed to the Merciful to abstain — that is, from speaking.” What is meant here is abstaining from food, drink, and sexual intercourse from dawn until sunset with the explicit intention of doing so (for the sake of Allah).
Abu Hurairah reported the Messenger of Allah (peace be upon him) as saying: “Allah said: ‘Every action of the son of Adam is for him except fasting, for that is solely for Me. I give the reward for it.‘ The fast is a shield. If one is fasting, he should not use foul language, raise his voice, or behave foolishly. If someone reviles him or fights with him he should say, ‘I am fasting,’ twice. By the One in whose hand is the soul of Muhammad, the [bad] breath of the one who is fasting is better in the sight of Allah on the Day of Resurrection than the smell of musk. The one who is fasting is happy at two times: when he breaks his fast he is happy with it, and when he meets his Lord he will be happy that he has fasted.” (Related by Ahmad, Muslim, and An-Nasa’i).
Al-Bukhari and Abu Dawud added in another version: “He leaves his food, drink, and desires for My sake. His fasting is for Me… I will give the reward for it, and for every good deed, he will receive 10 similar to it.”
Sahl Ibn Sa’d reported that the Prophet (peace be upon him) said: “There is a gate to Paradise that is called Ar-Rayyan. On the Day of Resurrection it will say: ‘Where are those who fasted?’ When the last [one] has passed through the gate, it will be locked.” (Reported by Al-Bukhari and Muslim)