«فَمَنْ شَهِدَ مِنْكُمُ الشَّهْرَ فَلْيَصُمْهُ»
Whoever among you witnesses the month should fast it.
The expression al-shahr may be understood with reference to the sighting or commencement of the month, while the pronoun -hu concerns the fasting period as a whole. The textbook treats this difference between an expression’s immediate sense and the sense carried forward by its pronoun as istikhdam.
Grammatically, -hu is the object of the command to fast. The rhetorical point is the semantic shift within the antecedent relation.