All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Mentally challenged or retarded people are labeled as “ma‘tooh” in the Sharee'ah. In his book At-Tashree‘ Al-Jinaa’i (Criminal Law in Islam), Abdul-Qaadir ‘Oudah said: “Muslim jurists define the ‘ma‘tooh’ or imbecile as a person with a low level of comprehension, unintelligible speech, and impaired judgment, whether the condition was inborn or following an illness. It is deduced from this definition that imbecility (‘atah) is the least degree of insanity. It can be said that insanity leads to total loss of mind or mental disturbance whereas imbecility (‘atah) leads to mental weakness of varying degrees ...” [At-Tashree‘ Al-Jinaa’i]
The ma‘tooh person is treated like the minor and custody over him is established. The Kuwaiti Encyclopedia of Fiqh reads: “Muslim jurists arrived at a consensus that custody over a minor is established in the Sharee'ah and this is the opinion of the majority of Muslim scholars. Likewise, the majority of scholars - the Hanafis, Shaafi‘is, Hanbalis, and one of the reported opinions of the Maalikis - establish custody for the adult who is insane or ma‘tooh.”
There is a difference of opinion among scholars regarding who is worthier of taking custody of the insane or ma‘tooh adult and how long should custody be? There is no doubt that the criteria for determining custody is the best interest of the ma‘tooh person. The whole point of custody in the first place is catering to the needs of the ward and serving his best interests. If the father of the ma‘tooh has passed away and he has no wealth from which he can provide for himself, his relatives and those who are legislatively obliged to spend on him (had he been sane) should provide for him. Scholars held different opinions about that matter as well. In case of dispute, the Sharee'ah court is the concerned authority entitled to settle such a dispute. The verdict of the Muslim judge is binding and decisively ends the scholarly difference of opinion in such matters open for Ijtihaad (personal reasoning by competent scholars in matters untackled by the Quran and Sunnah).
It should be noted that Muslims are encouraged to devotedly care for this type of people given their weakness and need of help. In fact, one may be blessed with sustenance, victory, and success because of helping them. Sa‘d ibn Abi Waqqaas narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “You gain no victory or livelihood except through (the blessings and invocations of) the weak amongst you.” [Al-Bukhari]
Also, there is no harm in placing the ma‘tooh in a suitable home to care for him if necessary. For more benefit, please refer to Fatwa 206671.
Such people existed at the time of the Prophet, sallallaahu ‘alayhi wa sallam, and they received the needed care in that righteous community. Ibn ‘Umar narrated that Hibbaan ibn Munqith was a man who had suffered from a head injury and developed a mental disorder but that did not stop him from engaging in trade. He was always being cheated, so he went to the Prophet, sallallaahu ‘alayhi wa sallam, and told him about that. He, sallallaahu ‘alayhi wa sallam, said to him: ‘When you buy something, you have the choice for three nights. If you are pleased with it, keep it, and if you are displeased then return it.” [Al-Haakim, Al-Bayhaqi and others]
The Prophet, sallallaahu ‘alayhi wa sallam, taught his Companions that the ma‘tooh is incompetent for religious assignments and should not be held accountable for his actions. They relied on that fact in their judgment. It has been narrated that a woman who had committed adultery was brought to ‘Umar who gave orders that she should be stoned. ‘Ali seized her and let her go. ‘Umar was informed of it so he summoned ‘Ali. ‘Ali came to him and said: “Commander of the Believers, you know that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: 'There are three (people) whose actions are not recorded: ‘A child till he reaches puberty, a sleeper till he awakes, and a ma‘tooh till he is restored to reason.' This woman is the ma‘toohah of the tribe of so and so...” [Abu Daawood]
Allaah Knows best.