Musannaf Ibn Abi Shayba 36:62

Abu Bakr narrated to us, he said: Zayd ibn al-Hubab narrated to us, from Ibn Lahi'ah, he said: Yazid ibn 'Amr al-Mu'afiri informed me, he said: I heard Abu Thawr al-Fahmi saying: Abd al-Rahman ibn 'Udays al-Balawi came to us, and he was among those who pledged allegiance under the tree. He ascended the pulpit, praised Allah, and extolled Him, then he mentioned Uthman. Abu Thawr said: So I entered upon Uthman while he was besieged, and he said: "I am the fourth of Islam."