What is the correct way to pronounce the letter "ذ" (dhal) in "صراط الذين" (sirāṭ alladhīna) in Sūrat al-Fātiḥah, both aloud and silently, and is pronouncing it like the letter "د" (dāl) considered a clear mistake (laḥn jalī)?

The letter 'Dhāl' (ذ) is articulated from the tip of the tongue touching the tips of the upper incisors. If a reciter pronounces it as 'Dāl' (د), it is a clear mistake (laḥn jalī) that invalidates the recitation for one who is able to pronounce it correctly. So, if someone omits a letter or replaces a letter with another, while their tongue is sound, their recitation is not valid. However, for someone unable to pronounce it correctly due to an accent or a lisp, there is no blame on them. Their prayer is valid, and it is permissible for someone in a similar situation to follow them in prayer. The points of articulation of letters do not differ between silent and vocalized pronunciation.