What is the ruling on the prayer of the Imam and the congregants after the Imam made a mistake in the rukoo' (bowed twice) before sujood?

If the Imam adds an extra bowing (ruku') inadvertently, his prayer is valid, and he should perform the prostration of forgetfulness (sujud al-sahw) after the Taslim. The latecomer (masbuq) who joins the Imam in this extra bowing, his following is valid. However, this bowing does not count as a rak'ah for him, and he must perform another rak'ah after the Imam's Taslim. If the Imam returns to bowing while knowing it is forbidden, his prayer becomes void. If he was ignorant, it does not become void. The Imam should alert the congregants who joined him in this extra bowing so that they do not count it.