Musannaf Ibn Abi Shayba 1:220

Isma'il ibn 'Ulayyah and Ibn Numayr narrated to us, from Muhammad ibn Ishaq, from Yazid ibn Abi Habib, from Marthad ibn 'Abd Allah al-Yazani, from Humayd ibn Ghusaylah al-Sunabihi, who said: I saw Abu Bakr wiping over the khimar (head covering).