Musnad Ahmad 8:14

Sulayman ibn Dawud al-Hashimi narrated to us, Isma'il—meaning Ibn Ja'far—narrated to us, Musa ibn 'Uqbah informed me, from Abu al-Nadr, the freed slave of 'Umar ibn 'Ubayd Allah ibn Ma'mar, from Abu Salamah ibn 'Abd al-Rahman, from Sa'd ibn Abi Waqqas: That the Prophet, peace and blessings of Allah be upon him, said regarding wiping over the leather socks (khuffayn): "There is no harm in that."