Sahih al-Bukhari — Hadith 1904
Narrated by Abu Hurairah
قَالَ اللَّهُ كُلُّ عَمَلِ ابْنِ آدَمَ لَهُ إِلاَّ الصِّيَامَ، فَإِنَّهُ لِي، وَأَنَا أَجْزِي بِهِ
Translation
The Prophet said: 'Allah said: All the deeds of Adam's sons are for them, except fasting which is for Me, and I will give the reward for it. Fasting is a shield from the fire and from committing sins. If one of you is fasting, he should avoid sexual relation with his wife and quarreling. By Him in Whose Hands my soul is, the unpleasant smell coming from the mouth of a fasting person is better in the sight of Allah than the smell of musk. There are two pleasures for the fasting person: one at the time of breaking his fast, and the other at the time when he will meet his Lord.'
Commentary
This is a Hadith Qudsi — a divine saying transmitted through the Prophet. God claims fasting as uniquely His because, unlike prayer or charity which can be observed by others, fasting is an entirely private act between a person and God. No one can verify whether someone is truly fasting — making it the purest expression of sincerity. The promise that God Himself will determine its reward (rather than specifying a fixed reward) suggests that fasting's reward is limitless. The mention of the fasting person's breath being 'better than musk' to God further elevates this act of self-denial into something beautiful in the divine estimation.