The Messenger of Allah ﷺ said, "Every deed of the son of Adam is for him, except fasting. It is for Me, and I will reward it. Fasting is a shield. When it is the day of fasting for one of you, let him not speak obscenely or raise his voice. If someone insults him or tries to fight him, let him say, 'I am fasting.' By the One in whose hand is the soul of Muhammad, the smell coming from the mouth of a fasting person is sweeter to Allah on the Day of Resurrection than the fragrance of musk. The fasting person has two joys he rejoices in. When he breaks his fast, he rejoices at breaking it, and when he meets his Lord , he rejoices at his fasting."
أخبرني إبراهيم بن الحسن عن حجاج قال قال ابن جريج أخبرني عطاء عن أبي صالح الزيات أنه سمع
قال رسول الله ﷺ كل عمل ابن آدم له إلا الصيام هو لي وأنا أجزي به والصيام جنة إذا كان يوم صيام أحدكم فلا يرفث ولا يصخب فإن شاتمه أحد أو قاتله فليقل إني صائم والذي نفس محمد بيده لخلوف فم الصائم أطيب عند الله يوم القيامة من ريح المسك للصائم فرحتان يفرحهما إذا أفطر فرح بفطره وإذا لقي ربه عز وجل فرح بصومه
Bukhari transmitted it, in the Book of Fasting, chapter "Should he say, 'I am fasting,' when he is cursed," hadith no. 1904, and Muslim, hadith no. 1151/115 (see the previous hadith, from Ibn Jurayj, through the same route), and it is in Kubra, hadith no. 2526.