Sahih Bukhari Volume 8, Book 73, Hadith Number 40.

Narated By An-Nu’man bin Bashir : Allah’s Apostle said, “You see the believers as regards their being merciful among themselves and showing love among themselves and being kind, resembling one body, so that, if any part of the body is not well then the whole body shares the sleeplessness (insomnia) and fever with it.”