Sahih Muslim Book 20, Hadith Number 4697.

Chapter : Beware of those who could not join a jihad expedition on account of illness or some other excuse.

It has been narrated on the authority of Jabir who said: We were with the Holy Prophet (may peace be upon him) on an expedition. He said: There are some people in Medina. They are with you whenever you cover a distance or cross a valley. They have been detained by illness.