Sahih Muslim Book 20, Hadith Number 4677.

Chapter : Jihad not compulsory for those who have a genuine excuse.

It has been narrated on the authority of Bara’ who said: When the Qur’anic verse: “Those who sit (at home) from among mu’min” (iv. 94) was revealed, the son of Umm Maktum spoke to him (the Holy Prophet). (At this), the words: “other than those who have a trouble (illness)” were revealed.