The bond that Allah has set amongst the believers
Kitāb al-Muʾmin|Volume 1|Book 1|Chapter 3
90 - وعن أبي عبد الله (ع) قال لا والله، لا يكونُ المؤمن مؤمناً أبداً حتىّ يكونَ لأخيه مثل الجسدِ إذا ضرب عليه عرقٌ واحد تداعت له سائرُ عروقه.
90. It has been reported that Abu 'Abd Allah عليه السلام said: Nay, by Allah! A believer is not a true believer ever until he acts as the body of his brother; when one vein in it is struck, it evokes the rest of the body.