What is the ruling of Islamic law regarding the prayer of the Imam and the followers in the event that the Imam forgets the first Tashahhud in the Asr prayer, then sits for the Tashahhud and the worshippers follow him, then he stands up for the fourth Rak'ah, then he stands up for the fifth Rak'ah thinking it was the third, then performs the two prostrations of forgetfulness and the Tasleem?

Whoever knows about an extra action (increase) by the Imam in his prayer and continues with him, then his prayer is invalid and he must repeat it. As for the one who did not know until he finished the prayer, then his prayer is valid. And the Imam, if he knows about the extra Rak'ah (unit of prayer) and continues, then his prayer is invalid. But if he believed his prayer was valid until he finished it, then his prayer is valid. And if he is informed about the increase and is not sure, then he must perform the prostrations of forgetfulness (Sujud (definition) as-Sahw).