Say, "Have you considered: if the Qur'an was from Allah , and you disbelieved in it while a witness from the Children of Israel has testified to something similar and believed while you were arrogant... ?" Indeed, Allah does not guide the wrongdoing people.
According my understanding, an Israelite testified something from the Qur'an as truth against unbelievers.
Who was he and what exactly did he testify for? Or is my understanding about this verse incomplete/incorrect?
The testifier was Moses/Musa (peace be upon him) and the thing which is similar to the Quran is the Torah.
The testifier was Abdullah ibn Salaam (May Allah be pleased with him) whos testimony was by believing it and saying it is the word of Allah and again the similar thing to the Quran is the Torah. Those who said so added the hadith of Sa'ad ibn abi Waqas as an evidence for it. This interpretation is supported by al-Qurtobi and ibn Kathir, who supported this opinion and added that this also applies to others (as many other Jews converted to Islam at the time of the Prophet (peace be upon him)), but he emphasized statement of the hadith:
I have never heard the Prophet (ﷺ) saying about anybody walking on the earth that he is from the people of Paradise except `Abdullah bin Salam.
Ibn 'Ashur in his tafsir at-tahrir wa-tanwir was in favor for the last statement, that this applies to any of the Jews of that time, and consider it much higher and closer to truth.
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