Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4697
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.