In the name of Allah, Most Gracious, Most Merciful.
Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (SAWS) 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 (Lord).” (Al-Bukhari and Muslim).
In a narration by Al-Bukhari, the Messenger of Allah (SAWS) said: “Allah says: ‘(The person observing Saum) has abstained from food and drink, and sexual pleasures for My sake; fasting is for Me, and I will bestow its reward. Every good deed has ten times its reward’.”
In a narration by Muslim, the Messenger of Allah (SAWS) 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. The person observing Saum abstains from food and drink only for My sake.’ The fasting person has two joyous occasions, one at the time of breaking his fast, and the other at the time of meeting his Rubb (Lord). Surely, the breath of one observing Saum is better smelling to Allah than the fragrance of musk.”
SAWS: May Allah exalt his mention and protect him from imperfection
Commentary: “Khuluf”, is the smell that comes from the mouth of the person who observes Saum. It results from the thirst and hunger which the person has to suffer suring the Saum. The Hadith not only tells us the merits of Saum but also brings into focus the true significance of it. Saum is not thus mere abstention from food, drink and sexual intercourse, but also keeping away from all evils and vices and embracing virtues.
Riyaadiss-Saliheen (The Meadows of the Righteous)
By Imam An Nawawi
Chapter 217, Page 354, No 1215