Musannaf Ibn Abi Shayba 3:139

Zayd ibn al-Hubab narrated to us, from Rabi’, from Ibrahim al-Hanafi, who said: I asked Ibn Sirin about a man who comes to the Imam while he is bowing. He said: He should open the prayer with a Takbir, and he should say Takbir for the bowing. If he does not do that, it does not suffice him.