Musnad Abu Ya'la al-Mawsili 3:82

Ubayd Allah narrated to us, he said: Ghundar narrated to us, he said: Shu'bah narrated to us, from 'Amr ibn Murrah, from Sa'id ibn al-Musayyib, who said: 'Ali and 'Uthman met, and 'Uthman was forbidding the Mut'ah (temporary marriage or combining Hajj and 'Umrah) or forbidding the 'Umrah. 'Ali said: "What do you intend by a matter that the Messenger of Allah, peace and blessings of Allah be upon him, did, yet you forbid it?" 'Uthman said: "Leave us be, away from you." 'Ali said: "I cannot leave you be." He said: So when 'Ali saw that, he entered into the state of Ihram for both of them together.