Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates: I heard the Messenger of Allah (peace and blessings be upon him) saying: "Allah, the Mighty and Majestic, says: Every deed of the son of Adam is for him except fasting, for it is for Me and I alone will reward it. By the One in whose hand is the soul of Muhammad (peace and blessings be upon him), the smell of the fasting person's mouth is more pleasing to Allah Almighty than musk."
(In Sahih Muslim, the hadith of Ibn Wahb is narrated via Harmalah, in which it says that Allah Almighty says: "Every deed of the son of Adam.")
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8507