The Messenger of Allah صلى الله عليه وسلم  said, “Allah the Exalted and Majestic said: `Every act of the son of Adam is for him,  except As-Siyam (the fasting) which is (exclusively) for Me, and I will reward him for  it.’ Fasting is a shield. When anyone of you is observing fast, he should neither indulge in obscene language nor should he raise his voice; and if anyone reviles him or tries to quarrel with him, he should say: `I am fasting.’ By Him in Whose Hand the soul of Muhammad is, the breath of one observing Saum is sweeter to Allah than the fragrance of musk. The one who fasts, experiences two joys: he feels pleasure when he breaks the fast. He is joyful by virtue of his fast when he meets his Rubb.”

Al-Bukhari and Muslim. Narrated by Abu Hurairah.