Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) reports that the Prophet (peace and blessings be upon him) said: "The smell of the fasting person's mouth is purer with Allah Almighty than the fragrance of musk." "Allah Almighty says: He fasted for My sake and gave up the desire for food for My sake, so fasting is for Me, and I shall reward it."
Ahmad recorded it. Its chain contains Atiyyah bin Sa'd; there is much criticism of him, and he has been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5075