In laysa al-khabaru bi-ṣaḥīḥin, bāʾ is additional and emphatic. Ṣaḥīḥin is formally genitive but accusative in syntactic position as the predicate of laysa.
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Additional Bāʾ with Laysa
Additional Particles: Seven Forms, Emphasis and Syntactic Case
In laysa al-khabaru bi-ṣaḥīḥin, bāʾ is additional and emphatic. Ṣaḥīḥin is formally genitive but accusative in syntactic position as the predicate of laysa.