Musnad Abu Ya'la al-Mawsili 143:3

Abu Khaythama narrated to us, Waki' narrated to us, from Khalid bin 'Abd al-Rahman bin Bukayr al-Sulami, from Ghalib al-Qattan, from Bakr al-Muzani, from Anas, who said: When we prayed with the Prophet, may Allah bless him and grant him peace, we would prostrate upon our garments for fear of the heat.