"A Book whereof the verses have been detailed," meaning: explained with the lawful and the prohibited, and the command and the prohibition, "as an Arabic Quran for a people who know," meaning: who believe.
﴿كتاب فصلت﴾ أَي: فسرت ﴿آيَاتُهُ﴾ بِالْحَلَالِ وَالْحَرَامِ , وَالْأَمْرِ وَالنَّهْيِ ﴿قُرْآنًا عَرَبيا لقوم يعلمُونَ﴾ يُؤمنُونَ
