What is the ruling on following the Imam in the prostration of forgetfulness (Sujud al-Sahw), and what is the ruling on the prostration of forgetfulness for the congregant (Ma'mum)?

The congregant must follow their imam in the prostration of forgetfulness (sujud al-sahw) if they joined the imam for all the rak'ahs, based on the Prophet's (peace be upon him) saying: "Indeed, the imam is appointed to be followed." This applies whether the prostration is before or after the salutation (taslim).

As for the latecomer (masbuq), they follow their imam if the prostration occurs before the salutation. However, if the prostration occurs after the salutation, they do not follow the imam. Instead, they complete their prayer, say the salutation, then perform the prostration of forgetfulness, and say the salutation.

In detail:

1. If the congregant joined the entire prayer with the imam and the imam made a mistake: The congregant must follow the imam in the prostration of forgetfulness, whether it is before or after the salutation.

2. If the congregant is a latecomer and the imam made a mistake during the part of the prayer that the congregant joined: If the imam prostrates before the salutation, the congregant prostrates with him, then completes their prayer, and then prostrates for forgetfulness again. If the imam prostrates after the salutation, the latecomer does not prostrate with him. Instead, they complete their prayer, say the salutation, then prostrate for forgetfulness, and say the salutation.

3. If the congregant is a latecomer and the imam made a mistake during the part of the prayer that the congregant did not join: If the imam prostrates before the salutation, the congregant follows him, then completes their prayer, and is not required to prostrate again. If the imam prostrates after the salutation, the congregant does not follow him and is not required to prostrate.

4. If the congregant made a mistake in their prayer and was not a latecomer: There is no prostration for them because the imam bears it on their behalf, unless the mistake invalidates a rak'ah (such as forgetting Al-Fatiha). In this case, they must complete the rak'ah after the imam's salutation, then perform the tashahhud, say the salutation, and prostrate after the salutation.

5. If the congregant made a mistake in their prayer and was a latecomer: They must perform the prostration of forgetfulness, whether their mistake occurred while praying with the imam or after they stood up to complete what they missed.