مَرَرْتُ بِعَبْدِ اللَّهِ.
Translation: I passed by ʿAbd Allāh.
بِ: preposition.
عَبْدِ: genitive with kasra; also muḍāf.
اللَّهِ: muḍāf ilayh, genitive with kasra.
The first element takes kasra in the genitive; it is not treated as a diptote here.
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ʿAbd Allāh: Genitive Parsing
Diptote Causes — Compounding and Its Conditions
مَرَرْتُ بِعَبْدِ اللَّهِ.
Translation: I passed by ʿAbd Allāh.
بِ: preposition.
عَبْدِ: genitive with kasra; also muḍāf.
اللَّهِ: muḍāf ilayh, genitive with kasra.
The first element takes kasra in the genitive; it is not treated as a diptote here.