Musnad Ahmad 652:34

Ishaq ibn Yusuf narrated to us, he said: Zakariya narrated to us, from al-Sha’bi, from al-Nu’man ibn Bashir, from the Prophet, peace and blessings of Allah be upon him, that he said: "The likeness of the believers in their mutual love, their mutual compassion, and their mutual mercy is like the body; when one member of it complains, the rest of the body responds to it with sleeplessness and fever."