Musnad Abu Ya'la al-Mawsili 3:174

Ubayd Allah ibn Umar narrated to us, Ghundar narrated to us, Shu'bah narrated to us, from al-Hakam, from Ali ibn al-Husayn, from Marwan ibn al-Hakam, who said: I witnessed Ali and Uthman between Makkah and al-Madinah, and Uthman was forbidding the *mut'ah* (combining Hajj and Umrah) and he would not combine them. When Ali saw that, he entered into the state of *ihram* for both of them and said: "Labbayk for an Umrah and a Hajj together." Uthman said: "Do you see me forbidding the people while you do it?" Ali said: "I would not abandon the Sunnah of the Messenger of Allah (may Allah bless him and grant him peace) for the saying of anyone among the people."