The Prophet Muhammad (peace be upon him) said that Allah Almighty says: Every deed of the son of Adam is for him except fasting; it is for Me and I Myself will reward it. The fasting person leaves his food and drink for My sake, so his fast is for Me, and therefore I Myself will give its reward. Every good deed is multiplied tenfold up to seven hundred times, except fasting, for it is for Me and I alone will reward it.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 7494