An-Nu'man ibn Bashir reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "The metaphor of the believers in their mutual love, mercy and affection is that of the body. When one limb of it complains, the rest of the body collapses with sleeplessness and fever."

Riyadh-as-Saliheen by Imam Al-Nawawi, volume 2, hadith number 224 - Sunni Hadith