It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) said: Allah Almighty has said, "Every deed is an expiation, but fasting is especially for Me, and I Myself will reward it. The breath of the fasting person is more pleasant to Allah than the fragrance of musk."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9888