Musannaf Ibn Abi Shaybah

Hazrat Usman bin Aswad narrates that I and Amr bin Tamim entered the mosque. The Imam had gone into ruku, so I and he also performed ruku, then while in the state of ruku, we walked and joined the row. When we completed the prayer, Amr said to me, "From whom have you heard what you just did being called permissible?" I said, "From Mujahid," and he said that he had seen Hazrat Ibn Zubair do so.