قُلْ أَعُوذُ بِرَبِّ الْفَلَقِ
مِنْ شَرِّ مَا خَلَقَ
وَمِنْ شَرِّ غَاسِقٍ إِذَا وَقَبَ
وَمِنْ شَرِّ النَّفَّاثَاتِ فِي الْعُقَدِ
وَمِنْ شَرِّ حَاسِدٍ إِذَا حَسَدَ
Notice the recurring مِنْ شَرِّ. Its rule must stay consistent across occurrences, whereas غَاسِقٍ إِذَا gives tanwīn a different following consonant and therefore a different rule.
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Al-Falaq: the complete passage
Applied Tajwīd Diagnostics: Four Short Sūrahs
قُلْ أَعُوذُ بِرَبِّ الْفَلَقِ
مِنْ شَرِّ مَا خَلَقَ
وَمِنْ شَرِّ غَاسِقٍ إِذَا وَقَبَ
وَمِنْ شَرِّ النَّفَّاثَاتِ فِي الْعُقَدِ
وَمِنْ شَرِّ حَاسِدٍ إِذَا حَسَدَ
Notice the recurring مِنْ شَرِّ. Its rule must stay consistent across occurrences, whereas غَاسِقٍ إِذَا gives tanwīn a different following consonant and therefore a different rule.