Musnad Ahmad 1301:22

Hammad ibn Khalid narrated to us, he said: Hisham ibn Sa’d narrated to us, from Uthman ibn Hayyan and Isma’il ibn Ubayd Allah, from Umm al-Darda’, from Abu al-Darda’, who said: "We truly saw ourselves on some of our journeys, and one of us would place his hand upon his head from the intensity of the heat, and there was no one among the people fasting except the Messenger of Allah, peace and blessings of Allah be upon him, and Abdullah ibn Rawahah." And Abu ‘Amir said: Uthman ibn Hayyan alone.