وَبِٱلْحَقِّ أَنزَلْنَـٰهُ وَبِٱلْحَقِّ نَزَلَ ۗ وَمَآ أَرْسَلْنَـٰكَ إِلَّا مُبَشِّرًا وَنَذِيرًا Qur’an Al-Israa (17:105)Wabialhaqqi anzalnahu wabialhaqqi nazala wama arsalnaka illa mubashshiran wanatheeran
I.e., it has come down to man, through the Prophet, without any alteration, omission or addition.
Lit., "with truth" or "in truth".
The Qur-an was sent down by Allah in Truth: it was not forged by any mortal. It has descended in Truth: it was not and has not been falsified or corrupted in the process of being communicated to mankind.
The part of the Prophet was that of a Messenger: he was not responsible if the ungodly rejected it. He fulfilled his mission in promulgating and explaining it and leaving it as a legacy to the world.