Musannaf Ibn Abi Shayba 1:226

Narrated to us Abd al-Rahman ibn Mahdi, from Sufyan, from Imran ibn Muslim, from Suwayd ibn Ghafala, from Nabata, who said: I asked Umar ibn al-Khattab about wiping over the turban. He said: "If you wish, wipe over it, and if you wish, do not."