Musannaf Ibn Abi Shayba 3:188

Abu Bakr narrated to us, he said: Hafs narrated to us, from Layth, from Mujahid, who said: I prayed behind Umar ibn Abd al-Aziz, and I counted for him four or five glorifications (tasbihat) in the bowing (ruku), and five or six glorifications in the prostration (sujud).