The source example مِنْ رَبِّهِ وَالْمُؤْمِنُونَ places a kasrah-bearing bāʾ before the pronoun hāʾ and a vowelled wāw after it. The hāʾ has kasrah and is given ṣilah.
Examine the immediate sequence رَبِّهِ وَ, not the unrelated nūn in the preceding word مِنْ.
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Application: min rabbihi wal-muʾminūn
Rules of Hāʾ al-Ḍamīr: Vowels, Ṣilah and Exceptions
The source example مِنْ رَبِّهِ وَالْمُؤْمِنُونَ places a kasrah-bearing bāʾ before the pronoun hāʾ and a vowelled wāw after it. The hāʾ has kasrah and is given ṣilah.
Examine the immediate sequence رَبِّهِ وَ, not the unrelated nūn in the preceding word مِنْ.