The Messenger of Allah ﷺ said, "The smell from the mouth of the one fasting is more pleasing to Allah on the Day of Resurrection than the fragrance of musk."
حدثنا بهز حدثنا سليم يعني ابن حيان حدثنا سعيد يعني ابن ميناء
أن رسول الله ﷺ قال خلوف فم الصائم أطيب عند الله يوم القيامة من ريح المسك
Its isnad is sahih. It appears in Jami al-Masanid wal-Sunan 7:167, quoting from this very place. It has already passed with this wording, with the addition of "the Day of Resurrection," within a lengthy hadith, no. 7679, through the narration of Ataa, from Abu Salih Zayyat, from Abu Hurairah. It will come again right after this, through the narration of Bashir bin Nuhayk from Abu Hurairah. The Hafiz pointed out in Fath al-Bari 4:90 to that narration, the narration of Ataa from Abu Salih, in the version of Muslim, Ahmad, and Nasai. See also 7775 and 7904.