Al-Mu'jam al-Saghir

Sayyiduna Nu’man bin Bashir (may Allah be pleased with both of them) reports that the Messenger of Allah (peace and blessings be upon him) said: “The example of the mutual friendship and love among the believers is like a single body: when a part of it becomes ill, the whole body suffers from sleeplessness and fever. There is a piece of flesh in the body; when it is sound and healthy, the whole body remains safe, and when it becomes corrupt, the whole body becomes corrupt. Beware! That is the heart.”