Musannaf Ibn Abi Shayba 1:35

Abu al-Ahwas narrated to us, from Mansur, from Salim ibn Abi al-Ja'd, from Thawban, the freed slave of the Messenger of Allah (may Allah bless him and grant him peace), who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "No one maintains the ritual purification (wudu) except a believer."