Musnad Ahmad 3:33

Rawh narrated to us, Shu'bah narrated to us, from Qatadah, who said: I heard Abdullah ibn Shaqiq saying: Uthman used to forbid Mut'ah (temporary marriage or Hajj al-Tamattu'), while Ali would perform the Talbiyah for it. Uthman said something to him, so Ali said to him: "You know that the Messenger of Allah, peace and blessings of Allah be upon him, did that?" Uthman said: "Yes, but we were afraid." Shu'bah said: I said to Qatadah: "What was their fear?" He said: "I do not know."