And before it was the scripture of Moses to lead and as a mercy. And this is a confirming Book in an Arabic tongue to warn those who have wronged and as good tidings to the doers of good.
The Qur'an is Allah's True Speech and the Position of the Disbelievers and the Muslims towards it
Allah says,
قُلْ
(Say) meaning, `O Muhammad to these idolators who disbelieve in the Qur'an.'
أَرَءَيْتُمْ إِن كَانَ
(Tell me! If it was) meaning, this Qur'an.
مِنْ عِندِ اللَّهِ وَكَفَرْتُمْ بِهِ
(from Allah, and you disbelieved in it) meaning, `what do you think that Allah will do to you if this Book that I have come to you with is actually revealed to me from Him in order that I convey it to you, and yet you disbelieve in it and deny it
((at the same time), a witness from among the Children of Israel has testified to something similar) meaning, `the previous Scriptures that were revealed to the Prophets before me all testify to its truthfulness and authenticity. They have prophecied, well in advance, about things similar to that which this Qur'an informs of.' Concerning Allah's statement,
وَمِن قَبْلِهِ كِتَابُ مُوسَىٰ إِمَامًا وَرَحْمَةً (And before this, there was the Book of Musa, a guide and a mercy. - 46:12) This verse, for one thing, gives proof of the statement mentioned in verse 9 that the Holy Prophet ﷺ is not an unprecedented messenger and Qur'an is not a singular divine Book which may create doubts in the minds. Sayyidna Musa (علیہ السلام) came before him as a messenger, and Torah was revealed to him which is accepted by all these Jews and Christians. Secondly, it supports the verse 10 above, because Sayyidna Musa (علیہ السلام) and Torah themselves are witnesses to the Holy Prophet ﷺ being the messenger of Allah and the Qur'an being Allah's Book.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 11–13
Initially, of the people who became the companions of the Prophet of God, many were old and belonged to the slave class, for example, Bilal, ‘Ammar, Shu‘ayb, Khubbab, etc. At the same time, among those who had faith in him, there were also people who belonged to respectable families, such as Abu Bakr ibn Abi Qahafa, ‘Uthaman ibn ‘Affan, ‘Ali ibn Abi Talib, etc. But the Prophet’s opponents used to mention only the former and not the latter. The reason is that when anyone becomes biased against a particular individual, his attitude becomes one-sided. He ignores the latter’s virtues and mentions only those aspects of him which give him the opportunity to degrade him. Similarly, the call of the Prophet Muhammad was undoubtedly the same as that of all the previous prophets: he had come with an eternal truth. But his opponents, instead of calling it ‘a very old truth’, called it ‘a very old falsehood.’ This kind of injustice prevailed in former times and, even today, it is quite common.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.
