The word refers to the Book of Allah, and their following of the best of it means that they act upon what Allah has commanded them therein, and abstain from what Allah has forbidden them therein.
وَالْقَوْل كتاب اللَّه , واتِّباعهم لأحسنه أَن يعملوا بِمَا أَمرهم اللَّه بِهِ فِيهِ , وينتهوا عَمَّا نَهَاهُم الله عَنهُ فِيهِ.
