Musnad Ahmad 5:194

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Qatadah, who said: Abdullah ibn Shaqiq said: Uthman used to forbid Mut'ah (temporary marriage/Hajj al-Tamattu'), and Ali used to command it. So Uthman said to Ali: "You are such and such." Then Ali said: "Indeed, you know that we performed Mut'ah with the Messenger of Allah, peace and blessings of Allah be upon him." He (Uthman) said: "Yes, but we were afraid."