Sahih Bukhari Volume 2, Book 26, Hadith Number 640.

Narated By Said bin Al-Musaiyab : ‘Ali and ‘Uthman differed regarding Hajj-at-Tamattu’ while they were at ‘Usfan (a familiar place near Mecca). ‘Ali said, “I see you want to forbid people to do a thing that the Prophet did?” When ‘Ali saw that, he assumed Ihram for both Hajj and ‘Umra.