The Messenger of Allah ﷺ said, "Whoever fasts a single day for the sake of Allah, Allah will distance the Fire from his face by seventy years for that day."
حدثنا محمد بن رمح بن المهاجر أنبأنا الليث بن سعد عن ابن الهاد عن سهيل بن أبي صالح عن النعمان بن أبي عياش
قال رسول الله ﷺ من صام يوما في سبيل الله باعد الله بذلك اليوم النار من وجهه سبعين خريفا
Bukhari, jihad and expeditions, chapter on the merit of fasting for the sake of Allah, no. 3840, from the hadith of Yahya bin Saeed and Suhayl bin Abu Salih from Numan, and Muslim, fasting, chapter on the merit of fasting for the sake of Allah for one who can bear it without harm or neglect of a duty, no. 1153, from Muhammad bin Rumh, from the hadith of Numan, with the same wording.