It is narrated from Sayyiduna Wathilah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Fasting is a shield, and it is one of the fortresses of the believer. Every deed belongs to the one who does it except fasting. Allah, the Mighty and Majestic, says: Fasting is for Me, and I Myself will reward it."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18361