Musannaf Ibn Abi Shayba 3:254

Ubayd Allah narrated to us, from Uthman ibn al-Aswad, who said: I and Abdullah ibn Tamim entered the mosque, and the Imam bowed, so I and he bowed, and we walked while bowing until we entered the row. When we entered the row, he said to me: "By the One whom you heard this from just now, from whom did you hear it?" I said: "From Mujahid." He said: "I have seen Ibn al-Zubayr do it."