He said: {And they were witnesses to what they were doing to the believers}, meaning their burning of them with fire.
قَالَ: ﴿وَهُمْ عَلَى مَا يَفْعَلُونَ بِالْمُؤْمِنِينَ شُهُود﴾ من تحريقهم إيَّاهُم بالنَّار