Allah, the Exalted, says:
“O you who believe! the fasting is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqun (the pious). (Fasting) for a fixed number of days, but if any of you is ill or on a journey, the same number (should be made up) from other days. And as for those who can fast with difficulty, (e.g., an old man), they have (a choice either to fast or) to feed a Miskin (poor person) (for every day). But whoever does good of his own accord, it is better for him. And that you fast is better for you if only you know. The month of Ramadan in which was revealed the Qur’an, a guidance for mankind and clear proofs for the guidance and the criterion (between right and wrong). So whoever of you sights (the crescent on the first night of) the month (of Ramadan, i.e., is present at his home), he must fast that month, and whoever is ill or on a journey, the same number (of days which one did not fast must be made up) from other days.” (2:183-185)
Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah ﷺ 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].
In a narration by Al-Bukhari, the Messenger of Allah ﷺ 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 ﷺ said, “The reward of every (good) deed of a person is multiplied from ten to seven hundred times. Allah ﷻ 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. Surely, the breath of one observing Saum is better smelling to Allah than the fragrance of musk.”
Commentary: “Khuluf”, is the smell that comes from the mouth of person who observes Saum. It results from the thirst and hunger which he has to suffer during 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.
(Riyad Us-Saliheen, Virtues, Chapter 217, No. 1215)