﴿ وَإِذَا تُتْلَىٰ عَلَيْهِمْ ءَايَاتُنَا بَيِّنَٰتٍۢ ۙ قَالَ ٱلَّذِينَ لَا يَرْجُونَ لِقَآءَنَا ٱئْتِ بِقُرْءَانٍ غَيْرِ هَٰذَآ أَوْ بَدِّلْهُ ۚ قُلْ مَا يَكُونُ لِىٓ أَنْ أُبَدِّلَهُۥ مِن تِلْقَآئِ نَفْسِىٓ ۖ إِنْ أَتَّبِعُ إِلَّا مَا يُوحَىٰٓ إِلَىَّ ۖ إِنِّىٓ أَخَافُ إِنْ عَصَيْتُ رَبِّى عَذَابَ يَوْمٍ عَظِيمٍۢ ﴾

“AND [thus it is:] whenever Our messages are conveyed unto them in all their clarity, those who do not believe that they are destined to meet Us [are wont to] say, "Bring us a discourse other than this, or alter this one.” Say [O Prophet]: "It is not conceivable that I should alter it of my own volition; I only follow what is revealed to me. Behold, I would dread, were I [thus] to rebel against my Sustainer, the suffering [which would befall me] on that awesome Day [of Judgment]!"”

Please check ayah 10:16 for complete tafsir.