وَمَا صَاحِبُكُم بِمَجْنُونٍ
Sentence structure
مَا negates the nominal clause. صَاحِبُكُم is nominative with attached كُم as its genitive possessor, and بِمَجْنُونٍ is the predicate phrase strengthened by بِ.
Ayah summary
Your well-known companion, the Prophet ﷺ, is certainly not mad.
وَمَا — And not
Grammar: وَ resumes the discourse and مَا negates the clause without an undisputed dialectal parse.
صَاحِبُكُم — (is) your companion
Grammar: A nominative noun; attached كُم is a second-person plural genitive possessor.
بِمَجْنُونٍۢ — mad
Grammar: Emphatic بِ strengthens the negation; مَجْنُونٍ is genitive in form and occupies the predicate position.