Musannaf Ibn Abi Shayba 3:2183

Abu Bakr narrated to us, he said: Ibn al-Mubarak narrated to us, from Hisham, from al-Hasan, who said: If he has been preceded by some of the prayer and the Imam has made a mistake, he said: He prostrates with the Imam, then he stands up and completes [what he missed].