We set out with the Messenger of Allah ﷺ on one of his military expeditions during intense heat, so much so that one of us would place his hand on his head from the severity of the heat. None among us was fasting except the Messenger of Allah ﷺ and Abdullah bin Rawaha.
حدثنا مؤمل بن الفضل حدثنا الوليد حدثنا سعيد بن عبد العزيز حدثني إسمعيل بن عبيد الله حدثتني أم الدرداء
خرجنا مع رسول الله ﷺ في بعض غزواته في حر شديد حتى إن أحدنا ليضع يده على رأسه أو كفه على رأسه من شدة الحر ما فينا صائم إلا رسول الله ﷺ وعبد الله بن رواحة
Muslim narrated it in the book of fasting, chapter on the choice between fasting and breaking the fast while traveling, hadith no. 1122, from the hadith of Walid bin Muslim. Bukhari narrated it in the book of fasting, the chapter following the chapter "If someone fasts some days of Ramadan then travels," hadith no. 1945, from the hadith of Ismail bin Ubaydullah, with the same content.