Musannaf Ibn Abi Shayba 16:711

Abu Bakr narrated to us, he said: Waki' narrated to us, from Sufyan, from Khalid, from Abdullah ibn al-Harith, who said: The sandal of the Messenger of Allah, may Allah bless him and grant him peace, had two straps (qibalan), the thong (shirak) of which was doubled.