Surah Al-A'raf — Ayah 184 (7:184)
أَوَلَمۡ يَتَفَكَّرُواْۗ مَا بِصَاحِبِهِم مِّن جِنَّةٍۚ إِنۡ هُوَ إِلَّا نَذِيرٞ مُّبِينٌ ١٨٤
English translation (Saheeh International)
Then do they not give thought? There is in their companion [i.e., Muḥammad (ﷺ)] no madness. He is not but a clear warner.
Brief commentary
Do those who reject the signs of Allah and His Messenger not think about things? If they used their minds it would be clear to them that Muhammad (peace be upon him) is not crazy. He is a messenger from Allah, Who sent him to give a clear warning about the punishment of Allah.