Is it permissible to pray behind an imam who pronounces all his words nasally, or is it merely disliked?

The worshiper must pronounce the letters from their correct points of articulation and give them their due rights and attributes. It is not permissible to pray behind someone who makes a clear, meaning-altering or letter-changing mistake ( لحن جلي ) in Al-Fatihah. As for letters with close points of articulation, such as the Dhad (ض) and the Zha (ظ), some scholars have made concessions regarding them, deeming the prayer valid even with their substitution due to the difficulty of avoiding it. This is the choice of Shaykh al-Islam. However, Al-Nawawi favored invalidity. If the imam is unable to pronounce otherwise (like someone with a lisp or who pronounces letters through their nose, thereby altering them), the majority view is that his prayer is not valid except behind someone similar to him. The Malikis, however, held that his imamate is valid, though disliked, and this is a strong opinion. Nevertheless, it is safer to prioritize someone behind whom prayer is valid by consensus. If the imam does not substitute letters but exaggerates their elongation, then prayer behind him is valid, and he should be advised and taught the correct way of recitation.