It is narrated from Sayyiduna al-Nu'man bin Bashir (may Allah be pleased with him) that he (peace and blessings be upon him) said: "The example of the believers in their mutual love, kindness and compassion is that of a single body: when one limb of it suffers, the whole body becomes restless with sleeplessness and fever."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6430