وَهُوَ كُلُّ اسْمٍ نُسِبَ إِلَيْهِ شَيْءٌ بِوَاسِطَةِ حَرْفِ الْجَرِّ لَفْظًا، نَحْوُ: «مَرَرْتُ بِزَيْدٍ».
Translation: A noun may be related through an explicitly expressed preposition, as in “I passed by Zayd.”
Bi- is the overt preposition. It governs Zaydin in the genitive, signalled here by kasrah. Together they form a jārr wa-majrūr phrase.