Must I repeat the prayer if I prayed behind an Imam wearing a shirt that reveals what is beneath it, knowing that I did so out of forgetfulness?

1- Prayer is not valid, according to many scholars, if performed in a garment that reveals what is beneath it. Some scholars elaborate, deeming the prayer void if the skin can be seen without close scrutiny, and disliked but valid with a recommendation to repeat it if the visibility requires close scrutiny.

2- The Imam's prayer is valid if he covers his 'awrah (which is between the navel and the knee according to the majority of jurists) with trousers, even if they are transparent.

3- The prayer of the congregant is valid according to many scholars if he prays behind an Imam whose prayer is invalid, while the congregant is unaware or ignorant of the Imam's condition.