What should we do if the Imam prostrates in congregational prayer after rising from his bowing, without saying the Takbir?

It is obligatory for the congregants to follow the Imam if they are aware of his movements through sight or by hearing the takbir. Hearing the takbir is not a condition for the validity of following; rather, it is the awareness of the Imam's movements that is essential, and this is by consensus. So, if the Imam prostrates and his takbir is not heard, it is obligatory to follow him into prostration as soon as his movement is observed. If the Imam says the takbir silently, there is nothing wrong with that, because audibly pronouncing the takbir is supererogatory, not obligatory. As for the Imam completely omitting the takbir, there is nothing wrong with that according to the majority of scholars who consider the takbirs of transition to be supererogatory. However, the Hanbalis consider them obligatory; so, if the Imam forgets them, he must perform the prostration of forgetfulness, and if he intentionally omits them, his prayer is invalidated according to them.