Musnad Ahmad 3:4

Yahya narrated to us, from Ibn Harmalah, who said: I heard Sa’id—meaning Ibn al-Musayyib—say: Uthman went out for Hajj. When he was on some part of the road, it was said to Ali—may Allah be pleased with them both—that he (Uthman) had forbidden the *Tamattu’* (combining) of the Umrah with the Hajj. Ali said to his companions: "When he departs, then depart." So Ali and his companions entered into the state of *Ihram* for Umrah. Uthman did not speak to him about that. Ali said to him: "Was I not informed that you forbade the *Tamattu’*?" He said: "Yes." He (Ali) said: "Did you not hear the Messenger of Allah, peace and blessings of Allah be upon him, perform the *Tamattu’*?" He said: "Yes."