Musnad Ahmad 5:581

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Amr ibn Murrah, from Sa'id ibn al-Musayyib, who said: 'Ali and 'Uthman met at 'Usfan, and 'Uthman was forbidding the *mut'ah* (temporary marriage or combining Hajj and 'Umrah) and the *'Umrah*. 'Ali said: "What do you intend by a matter that the Messenger of Allah, may Allah bless him and grant him peace, performed, yet you forbid it?" 'Uthman said: "Leave us be."