Musannaf Ibn Abi Shayba 3:5486

Waki’ narrated to us, from Sulayman ibn al-Mughirah, from Humayd ibn Hilal al-‘Adawi, from the one who heard the Bedouin saying: I saw the Messenger of Allah (may Allah bless him and grant him peace) praying in two sandals made of cowhide.