From Sahih Bukhari (Hadith 1569):
Narrated Sa`id 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 (ﷺ) did?" When `Ali saw that, he assumed Ihram for both Hajj and `Umra
Reference: Bukhari 1569