Is my prayer valid, or must I repeat it, after the Imam made a mistake in the Dhuhr prayer by sitting for the Tashahhud in the third rak'ah, and did not perform the prostration of forgetfulness (sujood al-sahw) until three minutes after finishing the prayer, knowing that I intended to repeat the prayer between the tasleem and the prostration of forgetfulness?

The Imam and the congregants must repeat the prayer if they limited it to three rak'ahs and omitted the fourth, because the omission of a pillar of prayer cannot be compensated for by the prostration of forgetfulness (sujood as-sahw); rather, it must be performed. Whoever forgets a rak'ah must perform it and then prostrate for forgetfulness, unless a long time has passed, in which case he must repeat the entire prayer. The congregants were obliged to separate from the Imam and complete their prayer individually when he insisted on his error. Muslims must learn the rulings of their religion to avoid such mistakes.