I went to visit Uthman bin Abu As, and he called for milk to be brought. I said, "I am fasting."He replied, "I heard the Messenger of Allah ﷺ say, 'Fasting is a shield against the Fire, like the shield one of you has in battle.'"
أخبرنا علي بن الحسين قال حدثنا ابن أبي عدي عن ابن إسحق عن سعيد بن أبي هند
دخلت على عثمان بن أبي العاص فدعا بلبن فقلت إني صائم فقال سمعت رسول الله ﷺ يقول الصوم جنة من النار كجنة أحدكم من القتال
[Sahih] Ahmad reported it, 4/21, from the hadith of Muhammad bin Ishaq bin Yasar with this wording. Ibn Khuzayma graded it sahih, no. 1891, and Ibn Ishaq explicitly stated that he heard it directly, as recorded there. The hadith is also found in al-Kubra, no. 2540. See the preceding hadith.