The Quran

فَلَعَلَّكَ تارِكٌ بَعضَ ما يوحىٰ إِلَيكَ وَضائِقٌ بِهِ صَدرُكَ أَن يَقولوا لَولا أُنزِلَ عَلَيهِ كَنزٌ أَو جاءَ مَعَهُ مَلَكٌ ۚ إِنَّما أَنتَ نَذيرٌ ۚ وَاللَّهُ عَلىٰ كُلِّ شَيءٍ وَكيلٌ

(Saheeh International)

Then would you possibly leave [out] some of what is revealed to you,¹ or is your breast constrained by it because they say, "Why has there not been sent down to him a treasure or come with him an angel?" But you are only a warner. And Allāh is Disposer of all things.

Knowing of the Prophet's difficulties, Allāh (subḥānahu wa ta‘ālā) urges him to patience, certain that he would not fail to convey the message in its entirety.