What is the ruling on the prayer of a congregant who joined the Imam in the second rak'ah, and when the Imam made the taslim after three rak'ahs, the congregant completed his prayer and performed the prostration of forgetfulness, while the Imam completed one rak'ah without performing the prostration of forgetfulness, claiming that his prayer would be invalidated according to the Hanafis if he forgot and made the taslim?

If the Imam says the salām forgetfully, his prayer is not invalidated, and he must complete his prayer and perform the prostration of forgetfulness (sujood as-sahw) if the interval is not long. Otherwise, his prayer and the prayer of any congregants who have not completed their prayer become invalid.

If the Imam returns to the prayer after having said the salām forgetfully, whether to complete it or to perform the prostration of forgetfulness, the congregant must either follow him or intend to separate (from the Imam). Otherwise, the congregant's prayer becomes invalid.

The Hanafis hold that the Imam's saying the salām forgetfully does not invalidate the prayer. He must complete it and perform the prostration of forgetfulness after the salām. The prostration of forgetfulness is waived if the interval becomes long.

If the congregant does not follow his Imam and does not intend to separate, his prayer is not valid and he must repeat it, because what he performed without following or intending to separate is not counted.