وَما صاحِبُكُم بِمَجنونٍ
Your companion is not mad.
EXPOSITION
This verse and the next three verses are all predicates (muqsam ʿalayhi or jawāb al-qasam) of the previous oaths.
There are six verses in the Quran apart from this verse which corroborate either directly or indirectly that Prophet Muhammad (s) was accused of being a madman by his contemporaries. These are: 15:6, 37:36, 44:14, 52:29, 68:2 and 68:51. This verse not only refutes the accusation of insanity imputed to the Prophet but also describes him as someone who was very intimately known to his contemporaries. This it does by the use of the word ṣāḥib, which means a close companion. This word is intended to convey the fact that the Prophet (Your companion) lives among you, and he has been Your companion throughout his life, and you are most knowledgeable about him, and you found him possessing intellectual integrity, sobriety in his opinions, and truthfulness in his speech, and so a person possessing such qualities is not accused of insanity. This accusation too is not novel with respect to the office of prophethood. The Quran provides several more references to similar accusations hurled at past prophets of God, such as 51:52 and 54:9, where Prophet Noah (a) is depicted as being accused of insanity by his people, and 26:27 and 51:39 where Prophet Moses (a) is depicted as being accused of the same charge.
[1] Tabrisi, 10/676.
[2] Mizan, 20/218.