Musannaf Ibn Abi Shayba 3:2190

Ibn Fudayl narrated to us, from Uqbah ibn Abi al-Ayzar, who said: I asked Ibrahim about a man who enters [the prayer] with the Imam, and the Imam has already preceded him by one rak'ah, and the Imam has made a mistake (saha), so what should he do? He said: When you enter with the Imam, do as he does.