We were on one of our journeys, and one of us would place his hand on his head from the intensity of the heat. Not a single person among the group was fasting except the Messenger of Allah ﷺ and Abdullah bin Rawaha.Abu Amir said, Uthman bin Hayyan alone.
حدثنا حماد بن خالد قال حدثنا هشام بن سعد عن عثمان بن حيان وإسماعيل بن عبيد الله عن أم الدرداء
لقد رأينا في بعض أسفارنا وإن أحدنا ليضع يده على رأسه من شدة الحر وما في القوم صائم إلا رسول الله ﷺ وعبد الله بن رواحة وقال أبو عامر عثمان بن حيان وحده
Its chain of transmission is sahih, despite some discussion regarding Uthman bin Hayan Dimashqi, the governor of Madinah in the days of Walid, though his hadith appears in Muslim. As for Ismail bin Ubaydullah, he is Makhzumi, a well-known trustworthy narrator, whose hadith appears in Sahihayn. This is why Ahmad paired him with Uthman. The hadith has already appeared at 21597.