What is the ruling on the prayer of the Imam and the congregants if the Imam forgot to vocalize the "takbeer" (Allahu Akbar) in the second rak'ah of the Asr prayer, and bowed, rose, and prostrated without anyone following him, then remembered and completed his prayer, and the congregants followed him? And what is required to be done in this situation?

The Imam's audible declaration of the takbir for transitioning between prayer postures is one of the Sunnahs of prayer. If one inadvertently omits it, some jurists say that Sujood al-Sahw (prostration of forgetfulness) is required, while others say it is not obligatory. According to the view that Sujood al-Sahw is legislated in this instance, its omission does not invalidate the prayer. It is prescribed for the Imam to say Tasbeeh, and for the congregants to follow him. If the congregants perform the bowing (ruku') and rising and then find the Imam in prostration, their prayer is valid, and their lagging behind him due to ignorance does not invalidate the prayer. However, if they do not perform the bowing and rising at all, then the prayer is invalidated.