﴿ ٱلَّذِينَ ءَاتَيْنَٰهُمُ ٱلْكِتَٰبَ يَعْرِفُونَهُۥ كَمَا يَعْرِفُونَ أَبْنَآءَهُمُ ۘ ٱلَّذِينَ خَسِرُوٓا۟ أَنفُسَهُمْ فَهُمْ لَا يُؤْمِنُونَ ﴾

“They unto whom we have vouchsafed revelation aforetime know this as they know their own children; yet those [of them] who have squandered their own selves -it is they who refuse to believe.”

(Those unto whom We gave the Scripture) those unto who We gave the knowledge of the Torah, i.e. 'Abdullah Ibn Salam and his followers (recognise (him)) Muhammad through his traits and description (as they recognise their sons. Those who ruin their own souls) through losing this world and the Next, i.e. Ka'b Ibn al-Ashraf and his followers (will not believe) in Muhammad or the Qur'an.