Our Imam pronounces the letter "Ṭa" as "Ḍad." So, what is the ruling concerning his imamate?

A person with a lisp (althagh) who substitutes one letter for another, falls into several categories:

1. Slight Lisp: If it does not prevent the original articulation point (makhraj) of the letter, then it does not cause harm, and his imamate (leading of prayer) is valid.

2. Severe Lisp with Ability to Correct: If he substitutes one letter for another and is able to correct his pronunciation but does not do so, then his prayer is invalid, as is his imamate, if the letter is in Al-Fatiha.

3. Severe Lisp with Inability to Correct: His prayer is valid by scholarly consensus. However, there is disagreement regarding the validity of his imamate. Many scholars hold that it is not valid, while others maintain its validity, citing the verse: "Allah does not charge a soul except [with that within] its capacity" (2:286), and by analogy with one who is unable to stand [in prayer]. Sheikh Ibn Uthaymeen holds that the imamate of a person with a lisp is valid as long as this is his capacity, but it is preferable to choose an imam who has no defect, in order to avoid dispute.

As for substituting the letter 'ṭāʾ' with 'ḍād', if it is a true substitution, then the aforementioned rules apply. If it is merely a slight proximity in pronunciation, then it is a slight lisp that does not cause harm.