Musannaf Ibn Abi Shayba 3:391

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from al-A'mash, from al-Musayyab ibn Rafi', from Zayd ibn Wahb, from 'Umar, who said: "If one of you is unable [to prostrate on the ground] due to the heat or the cold, then let him prostrate upon his garment."