Sahih Muslim Book 7, Hadith Number 2816.
Chapter : Permissibility of the Tamattu’ (form of pilgrimage).
Sa’id b. al-Musayyab reported that ‘Ali and ‘Uthman (Allah be pleased with them) met at ‘Usfan; and Uthman used to forbid (people) from performing Tamattu’ and ‘Umra (during the period of Hajj), whereupon ‘Ali said: What is your opinion about a matter which the Messenger of Allah (may peace be upon him) did but you forbid it? Thereupon Uthman said: You leave us alone, whereupon he (‘Ali) said: I cannot leave you alone. When ‘Ali saw this, he put on Ihram for both of them together (both for Hajj and ‘Umra).