Does failing to protrude the tip of the tongue when pronouncing the letter "dhāl" (ذ), or pronouncing it as a mix between "zāy" (ز) and "dhāl" (ذ) when reciting Al-Fatihah in prayer, invalidate it, and is it obligatory to repeat all previous prayers in which this occurred, and is there a consensus on this?

If a worshiper replaces the letter dhāl (ذ) with zāy (ز) in Al-Fātiḥah, their prayer is invalidated if they were able to learn and neglected to do so. However, if they were unable to learn, their prayer is valid, and it is not permissible for anyone to follow them in prayer except for someone who is in the same situation.

If a worshiper pronounces some letters incorrectly, for example, by saying "nasta‘īn" (نستعين) in a way that resembles the tā’ (ت) with a dāl (د) or the ṣād (ص) as a mix between ṣād and sīn (س), if they are unable to learn, their prayer is valid. But if they are able to learn, they must repeat every prayer in which they were negligent.

Whoever cannot distinguish between two letters must learn the difference between them. And whoever neglects to learn must make up for the prayers during the period of negligence. Learning the difference between two letters does not require significant effort.

The four schools of Islamic jurisprudence agree on the above. However, the later scholars of the Hanafi school believe that such an error does not invalidate the prayer if it was not intentional, due to the widespread nature of this issue among common people. This view has been adopted by some sheikhs and later scholars. Nevertheless, the opinion of the earlier scholars is more cautious.