Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "By the One in whose hand is my soul! The smell of the fasting person's mouth is more pleasing to Allah Almighty than the fragrance of musk. Allah, the Mighty and Majestic, says: He leaves his desire, his food and his drink only for My sake; fasting is for Me and I alone will reward it. Every good deed is multiplied from tenfold up to seven hundred times, except fasting, for it is for Me and I alone will reward it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8508