Musannaf Ibn Abi Shayba 3:4868

Abu Bakr narrated to us, he said: Jarir ibn Abd al-Hamid narrated to us, from Mansur, from Mujahid, who said: When Ibn al-Zubayr would stand in prayer, he was like a piece of wood due to [his] khushu (humility/devotion). Mujahid said: And it was narrated to me that Abu Bakr was like that.