Musnad Ahmad 652:87

Waki’ narrated to us, al-A’mash narrated to us, from al-Sha’bi, from al-Nu’man ibn Bashir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The believers are like one man; if his head complains, the rest of the body calls out to him with fever and sleeplessness."