Silsilat al-Ahadith al-Sahihah

Sayyiduna Nu’man bin Bashir (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: “Indeed, there is a piece of flesh in the body of the son of Adam; if it is sound, the whole body is sound, and if it is corrupt, the whole body becomes corrupt. Behold, that (piece) is the heart.”