Is it permissible to pray behind an imam who does not perform ablution properly, or does the impermissibility only apply if one witnesses him directly every time he performs ablution?

Whoever does not perform a sufficient ablution has an invalid prayer. The prayer of the congregant behind him is also invalid if the congregant is aware of his condition. However, if the congregant is unaware of his condition, or only becomes aware after the prayer has ended, his prayer is valid according to the preponderant opinion.

If you are not certain that the Imam did not fully wash the parts of his body during ablution, then it is permissible for you to pray behind him. However, if you are certain, then following him in prayer is forbidden. This also applies if such deficiency is his general habit in ablution.

The elbow, both its outer and inner parts, must be washed in ablution, based on the words of Allah Almighty: "O you who have believed, when you rise to [perform] prayer, wash your faces and your hands up to the elbows and wipe over your heads and wash your feet to the ankles." The word "up to" (إلى) in the verse means "including," and the entire elbow must be washed.

Scholars have differed regarding the sufficient amount for wiping the head. Some deem it sufficient to wipe only a part of the head, but wiping the entire head is more cautious.

The Imam should be advised gently, courteously, and with compassion regarding the obligation of washing the elbows completely, both their outer and inner parts.