What should the congregant do if the imam recites a verse of prostration and then bows, thinking he will prostrate, then the imam prostrates after being alerted, knowing that the congregant bowed with the imam? And what should the congregant who has not yet bowed do?

What the Imam did, by moving to the prostration of recitation (Sujud al-Tilawah) after forgetting it and performing Ruku' (bowing), is in agreement with the opinion of some Hanafis and Malikis. The Imam was therefore correct, and it is obligatory for the questioner and the congregants to follow him in the prostration of recitation, and then in the second bowing, which is the original bowing.