What is the ruling on the prayer of one who sat for the Tashahhud mistakenly after the first رکعه, then stood up before saying “Subhanallah” for it, and performed the prostration of forgetfulness (Sujud al-Sahw) before the Taslim? And what is the ruling on the prayer of one who did not follow the Imam in the prostration of forgetfulness, or one who did not make a mistake with the Imam but then prostrated with him for forgetfulness?

If the Imam sits after the first rak'ah for a period equivalent to the time it takes to recite the tashahhud (definition), he must perform the prostration of forgetfulness (sujud (definition) as-sahw). The congregants are obliged to follow him in this; if a congregant intentionally fails to follow, their prayer becomes invalid. However, if the Imam sits for a period less than that of the tashahhud, there are two opinions: either the prostration of forgetfulness is obligatory or it is not.