The Messenger of Allah ﷺ said, "No servant fasts a single day in the path of Allah except that Allah, by virtue of that day, distances the Fire from his face by seventy autumns."
حدثنا ابن نمير حدثنا سفيان عن سمي عن النعمان بن أبي عياش الزرقي
قال رسول الله ﷺ لا يصوم عبد يوما في سبيل الله إلا باعد الله بذلك اليوم النار عن وجهه سبعين خريفا
Its isnad is sahih. Ibn Numayr is Abdullah, and Sufyan is Ibn Uyayna, and Sumayy is the freedman of Abu Bakr bin Abdul-Rahman, and Numan is one of the senior sons of the companions, and all of them are trustworthy. Tirmidhi related the hadith, 4/166, no. 1623, in the chapters on the virtues of jihad, what has been said on the virtue of fasting, and Nasai, 4/174, no. 2251, in the chapter on fasting.