The Messenger of Allah ﷺ said, "The smell of the fasting person's breath is more pleasing to Allah on the Day of Resurrection than the fragrance of musk."
حدثنا عفان قال حدثنا سليم قال حدثنا سعيد قال سمعت
قال رسول الله ﷺ لخلوف فم الصائم أطيب عند الله يوم القيامة من ريح المسك
Its isnad is sahih, and it is part of a hadith that has appeared frequently, related by Bukhari 7/211 in "Clothing," "What is mentioned about musk," and Muslim 2/807 no. 1151, in "Fasting," "The merit of fasting."