I heard the Messenger of Allah ﷺ say, "Fasting is a shield against the Fire, like the shield one of you has in battle."
حدثنا محمد بن رمح المصري أنبأنا الليث بن سعد عن يزيد بن أبي حبيب عن سعيد بن أبي هند أن مطرفا من بني عامر بن صعصعة حدثه أن عثمان بن أبي العاص الثقفي دعا له بلبن يسقيه قال مطرف إني صائم
سمعت رسول الله ﷺ يقول الصيام جنة من النار كجنة أحدكم من القتال
[Its isnad is sahih] Reported by Nasai, 4/167, "Fasting," under "Mention of the disagreement regarding Muhammad bin Abu Yaqub," in the hadith of Abu Umama on the merit of the one who fasts, no. 2232, from the hadith of Layth with this wording. Ibn Khuzayma graded it sahih, no. 2125, and added, "and fasting three days from every month is a good fast." Munziri indicated that it is hasan. Ibn Hibban graded it sahih (Mawarid), no. 931. The hadith has another route in Nasai, 4/167.