Musannaf Ibn Abi Shayba 3:227

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Yunus, from al-Hasan, and from al-Mughirah, from Ibrahim, regarding the man who reaches the Imam while he is prostrating: They both said: He should follow him and prostrate with him, and he should not oppose him, and he should not count the prostration [as a rak'ah] unless he catches the bowing.