Musannaf Ibn Abi Shayba 3:245

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Mansur, from Zayd ibn Wahb, who said: I went out with Abdullah from his house to the mosque. When we reached the middle of the mosque, the Imam bowed, so Abdullah said the takbir, then he bowed, and I bowed with him. Then we walked while bowing until we reached the row, until the people raised their heads. He said: When the Imam finished the prayer, I stood up, and I thought that I had not caught it. Abdullah took my hand and made me sit down, and said: "You have indeed caught it."