"So let not their speech grieve you," meaning: that you are a sorcerer, or a poet, [or a soothsayer], or that you are insane, or a liar. "Indeed, We know what they conceal," of their enmity toward you, "and what they proclaim." Thus, Allah protects you from them and humbles them before you, and Allah did indeed do so to them.
Interpretation of Surah Yasin, verses 78 through 83.
﴿فَلَا يحزنك قَوْلهم﴾ إِنَّكَ سَاحِرٌ، وَإِنَّكَ شَاعِرٌ [وَإِنَّكَ كَاهِنٌ] وَأَنَّكَ مَجْنُونٌ، وَأَنَّكَ كَاذِبٌ ﴿إِنَّا نعلم مَا يسرون﴾ من عداوتهم لَك ﴿وَمَا يُعْلِنُونَ﴾ فيعصمك اللَّه مِنْهُم ويذلهم لَك، فَفعل الله ذَلِك بِهِ.
تَفْسِير سُورَة يس الْآيَات من آيَة ٧٨ إِلَى آيَة ٨٣.