The Evidence of the Truthfulness of the Quran:
Allāh Glory be to Him tells us
قُل لَّوْ شَاء اللّهُ مَا تَلَوْتُهُ عَلَيْكُمْ وَلَا أَدْرَاكُم بِهِ
Say:If Allāh had so willed, I should not have recited it to you nor would He have made it known to you… [Surah Yunus, 10:16]
This indicates that Prophet Muhammad ﷺ brought this Qur’ān only with the permission and will of Allāh Glory be to Him for him to do so. The proof of this was that he had not fabricated it himself and that they were incapable of refuting it, and that they should be fully aware of his truthfulness and honesty since he grew up among them, until Allāh Glory be to Him sent the Message to him.
The Prophet ﷺ was never criticized for anything or held in contempt. So he said,
فَقَدْ لَبِثْتُ فِيكُمْ عُمُرًا مِّن قَبْلِهِ أَفَلَ تَعْقِلُونَ
Verily, I have stayed among you a lifetime before this. Have you then no sense! [Surah Yunus, 10:16]
which means don’t you have brains with which you may distinguish the truth from falsehood?!
When Heraclius 1, the Roman king, asked Abu Sufyan and those who were in his company about the Prophet,
Heraclius said, ‘Have you ever accused him of telling lies before his claim (to be a Prophet)?’ I replied, ‘No.’ – al-Bukhari 2
Abu Sufyan was then the head of the disbelievers and the leader of the idolators, but he still admitted the truth. This is a clear and irrefutable testimony since it came from the enemy.
Heraclius then said
So I wondered how a person who does not tell a lie about others could ever tell a lie about Allāh – al-Bukhari 3
Jafar bin Abu Talib said to An-Najashi, the king of Ethiopia: Allāh has sent to us a Messenger that we know his truthfulness, ancestral lineage, and honesty. He stayed among us before the Prophethood for forty years.
Lastly, Allāh Glory be to Him challenged anyone who is doubtful of this book, Qur’ān.
أَمْ يَقُولُونَ ٱفْتَرَىٰهُ ۖ قُلْ فَأْتُوا۟ بِسُورَةٍ مِّثْلِهِۦ وَٱدْعُوا۟ مَنِ ٱسْتَطَعْتُم مِّن دُونِ ٱللَّهِ إِن كُنتُمْ صَٰدِقِينَ
Or do they say [about the Prophet], “He invented it?” Say, “Then bring forth a surah like it and call upon [for assistance] whomever you can besides Allāh, if you should be truthful.” [Surah Yunus, 10:38]