Sayyiduna Sahl bin Sa’d (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: “The relationship of a believer with the believers is like that of the head with the body; a believer is distressed by the suffering of the believers just as the head is distressed by the pain of the body.”
Explanation: It is narrated from Sayyiduna Nu'man bin Bashir radi Allahu anhu that the Messenger of Allah sallallahu alayhi wa sallam said: "The Muslims (in their mutual relations) are like a single person; if his eye is in pain, his whole body feels the pain, and if his head is in pain, his whole body feels the pain." [أخرجه مسلم : 2586]
That is, just as when any limb of the body aches, the whole body is affected, and merely the pain of one limb causes the entire body to be afflicted, in the same way, Muslims should also become like a single body, so that if any Muslim suffers harm, all Muslims share in his pain, and together they strive to remove his distress and hardship.