Musnad Ahmad 652:27

Yahya ibn Sa'id narrated to us, from Zakariyya, who said: 'Amir narrated to us, saying: I heard al-Nu'man ibn Bashir delivering a sermon, saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The likeness of the believers in their mutual love, their mercy, and their compassion is like the body; when one part of it complains, the rest of the body calls out to it with sleeplessness and fever."