كَمَا أَرْسَلْنَا إِلَى فِرْعَوْنَ رَسُولًا فَعَصَى فِرْعَوْنُ الرَّسُولَ
Just as We sent a messenger to Pharaoh, and Pharaoh disobeyed the messenger.
The first occurrence «رسولًا» introduces a messenger. The later definite form «الرسول» points back to that already-mentioned individual rather than introducing another person.
This is anaphoric or previously mentioned definiteness. The article ties the second mention to the first and maintains referential continuity.
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Al-Ta‘rif wa al-Tankir: Rhetorical Functions of Definiteness and Indefiniteness
كَمَا أَرْسَلْنَا إِلَى فِرْعَوْنَ رَسُولًا فَعَصَى فِرْعَوْنُ الرَّسُولَ
Just as We sent a messenger to Pharaoh, and Pharaoh disobeyed the messenger.
The first occurrence «رسولًا» introduces a messenger. The later definite form «الرسول» points back to that already-mentioned individual rather than introducing another person.
This is anaphoric or previously mentioned definiteness. The article ties the second mention to the first and maintains referential continuity.