Musnad Ahmad 8:21

'Affan narrated to us, Wuhaib narrated to us, Musa bin 'Uqbah narrated to us, he said: I heard Abu an-Nadr narrating from Abu Salamah, from Sa'd bin Abi Waqqas, a hadith which he attributed to the Prophet, peace and blessings of Allah be upon him, regarding wudu (wiping) over the leather socks: "There is no harm in it."