The Messenger of Allah ﷺ said, "Allah said, 'Every deed of the son of Adam is for him, except fasting. Fasting is for Me, and I alone shall reward it. The breath of the fasting person is sweeter to Allah than the scent of musk.'"
حدثنا عبد الرزاق حدثنا معمر وعبد الأعلى عن معمر عن الزهري عن ابن المسيب
قال رسول الله ﷺ قال الله عز وجل كل عمل ابن آدم له إلا الصيام الصيام لي وأنا أجزي به ولخلوف فم الصائم أطيب عند الله من ريح المسك
Its isnad is sahih. It is found in Jami al-Masanid 7:144-145, taken from this very passage. Its meaning has already appeared at length in 7679, through the narration of Abu Salih Zayyat from Abu Hurairah. Its meaning has also appeared, both at length and in brief, through other routes, which we pointed out there.