[Ruling on the Prayer of One Who Cannot Distinguish Between Ḍād (ض) and Ẓāʾ (ظ)]

Answer (and Allah grants success): Pronouncing ḍād perfectly is not easy for many—earlier and later generations alike. Al-Jāḥiẓ mentions in his book al-Bayān wa-t-Tabyīn matters concerning pronouncing ḍād and those who mastered it, from which one gathers evidence that pronouncing it as it ought is not widely attainable. On that basis, the prayer is valid, for “Allah does not burden a soul beyond its capacity.” [Al-Baqarah:286]
Indeed, I have seen reported from one of the masters of Arabic that some Arabs did not distinguish between ḍād and ẓāʾ, putting ẓāʾ in place of ḍād, and he adduced a line of their poetry; perhaps the variant readings in (ḍanīn/ẓanīn) in Sūrat at-Takwīr also support this—reports that would reduce the weight of treating such utterance as an error.
Imām al-Mahdī examined our companions’ statement that one’s prayer is invalid if he puts ẓāʾ in place of ḍād in “al-maghdūbi ʿalayhim wa-lā ḍ-ḍāllīn,” and he said: their points of articulation coincide; their case is not like that of ḥāʾ and khāʾ.
Imām Yaḥyā ibn Ḥamzah and al-Ghazālī said that exchanging one for the other does not invalidate the prayer; their wording—as in al-Baḥr "al-Imām Yaḥyā: “…except ḍād and ẓāʾ, due to their closeness.” End. This is transmitted in al-Bayān and its marginalia .
In Miftāḥ as-Saʿādah it is related that Imām Yaḥyā, Imām ʿIzz ad-Dīn, and ar-Rāzī said: no, due to the strong similarity between them: both are voiced, soft (riḫwah), and emphatic (muṭbaqah); and the ḍād, because of its softness, spreads such that it approaches the articulation of ẓāʾ.
They said: Because of this similarity, distinguishing between them is difficult, and only select experts and Qurʾān readers perceive it. Allah says: “He has not placed upon you in the religion any hardship.” [Al-Ḥajj:78] And the Prophet (May Allah bless him and his family and grant them peace) said: “I was sent with a ḥanīf, easy religion.”
What also supports this is the report, “I am the most eloquent of those who pronounce ḍād,” which indicates the variance among people in pronouncing it from its proper articulation. End.
What further reduces the weight of treating it as error is that, for example, the dialect of the people of Yemen does not distinguish between ḍād and ẓāʾ; changes arising from dialectal differences are overlooked when transmitting others’ speech—this is undisputed in human speech.
As for the speech of Allah, the apparent sense of the canonical readings shows toleration of such matters; the Arabs recited the Qurʾān each in his dialect and tongue, and this was not denounced.
It also lessens the severity that if ḍād is replaced with ẓāʾ, only the trained ear usually notices—given the two letters’ similarity—so the meaning is not impaired. This differs from replacing ḍād with dāl or qāf in ad-ḍāllīn, saying al-qāllīn or ad-dāllīn, which does shift the listener’s mind to a different meaning.
I have seen someone who read the Qurʾān well and mastered it yet did not bring ḍād from its precise articulation—while thinking he did. This too reduces the severity of counting it as error.
Therefore, the prayer behind someone who cannot perfectly distinguish the two letters is valid.
As for the laypeople’s prayer with obvious laḥn: the school holds—as in the marginalia—that if the layperson has not bound himself to a school, his prayer is valid, for it coincides with some ijtihād1.
Elsewhere in the marginalia the scholars said: If a layperson does not know the conditions of taqlīd, his taqlīd is no taqlīd, and his ruling is that of one with no school. End.
In the marginalia for the school: If the layperson has adopted a school and knows the conditions for valid taqlīd, and then matches the school he follows—there is no issue. If he does not match it: If he knows of the contravention, the prayer is no prayer—he repeats it within the time and makes it up after. If he is ignorant of the contravention, he repeats within the time but not after .
I say: Their statement—“If he knows of the contravention, he repeats within the time and makes up after”—should be limited to one who can rid himself of the laḥn. As for one who cannot, Allah does not burden a soul except [with that within] its capacity.
Among what the scholars of the school tolerated, they said does not invalidate the prayer: adding tanwīn at a pause, omitting tanwīn in liaison, not fully lengthening the vowels (the correct is: the ḥarakāt)—so in the marginal note; likewise shortening where lengthening is due and vice versa; likewise cutting a hamzat al-waṣl, but if he joins a hamzat al-qaṭʿ, the prayer is invalid. End of what is in the marginalia for the school.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1