Sahih Bukhari | Volume 8 | Hadith Number 40

Narrated 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."