In عَنْ صَلَاتِهِمْ, nūn sākinah is followed by ṣād, one of the ikhfāʾ letters. Therefore nūn is concealed with ghunnah rather than fully pronounced or fully assimilated.
Preserve the emphatic character of ṣād and the moderate nasal duration. Contrast an imaginary nūn followed by ʿayn: that would call for throat iẓhār instead. The next letter is the reason for the rule.
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ʿAn ṣalātihim: why ikhfāʾ applies
Al-Māʿūn and Al-Humazah: Applied Assimilation, Concealment, Madd and Hāʾ Rules
In عَنْ صَلَاتِهِمْ, nūn sākinah is followed by ṣād, one of the ikhfāʾ letters. Therefore nūn is concealed with ghunnah rather than fully pronounced or fully assimilated.
Preserve the emphatic character of ṣād and the moderate nasal duration. Contrast an imaginary nūn followed by ʿayn: that would call for throat iẓhār instead. The next letter is the reason for the rule.