from bukhari

Volume 2, Book 26, Number 634:

Narrated Marwan bin Al-Hakam:
I saw ‘Uthman and ‘Ali. ‘Uthman used to forbid people to perform Hajj-at-Tamattu’ and Hajj-al-Qiran (Hajj and ‘Umra together), and when ‘Ali saw (this act of ‘Uthman), he assumed Ihram for Hajj and ‘Umra together saying, “Lubbaik for ‘Umra and Hajj,” and said, “I will not leave the tradition of the Prophet on the saying of somebody.”

Volume 2, Book 26, Number 640:

Narrated 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.