Allah tells us about the obstinance of the disbelievers of the Quraysh, who were opposed to the message and denied Allah. When the Messenger ﷺ read to them from the Book of Allah and His clear evidence they said to him: "Bring a Qur'an other than this." They wanted the Prophet to take back this Book and bring them another book of a different style or change it to a different form. So Allah said to His Prophet :

قُلْ مَا يَكُونُ لِى أَنْ أُبَدِّلَهُ مِن تِلْقَآءِ نَفْسِى

(Say: "It is not for me to change it on my own accord;) This means that it is not up to me to do such a thing. I am but a servant who receives commands. I am a Messenger conveying from Allah.

إِنْ أَتَّبِعُ إِلاَّ مَا يُوحَى إِلَىَّ إِنِّى أَخَافُ إِنْ عَصَيْتُ رَبِّى عَذَابَ يَوْمٍ عَظِيمٍ

(I only follow that which is revealed unto me. Verily, I fear the torment of the Great Day (the Day of Resurrection) if I were to disobey my Lord.)

The Evidence of the Truthfulness of the Qur'an Muhammad ﷺ then argued with supporting evidence to the truthfulness of what he had brought them:

قُل لَّوْ شَآءَ اللَّهُ مَا تَلَوْتُهُ عَلَيْكُمْ وَلاَ أَدْرَاكُمْ بِهِ

(Say: "If Allah had so willed, I should not have recited it to you nor would He have made it known to you...") This indicates that he brought this only with the permission and will of Allah 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 Allah sent the Message to him. The Prophet was never criticized for anything or held in contempt. So he said,

فَقَدْ لَبِثْتُ فِيكُمْ عُمُراً مِّن قَبْلِهِ أَفَلاَ تَعْقِلُونَ

(Verily, I have stayed among you a lifetime before this. Have you then no sense) Which meant "don't you have brains with which you may distinguish the truth from falsehood" When Heraclius, the Roman king, asked Abu Sufyan and those who were in his company about the Prophet , he said: "Have you ever accused him of telling lies before his claim" Abu Sufyan replied: "No." 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: "I wondered how a person who does not tell a lie about others could ever tell a lie about Allah." Ja`far bin Abu Talib said to An-Najashi, the king of Ethiopia: "Allah 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."