Musannaf Ibn Abi Shaybah 2538 — Those who said two takbirs

Ibrahim al-Hanafi said: I asked Ibn Sirin about a person who joins the congregation while the imam is bowing. He said: He should say the opening takbir (takbirat al-ihram) to enter the prayer, and then say the takbir and join the bowing. If he does not do so, his prayer is not valid.