Musnad Ahmad 652:66

Sufyan narrated to us, from Mujalid, narrated to us al-Sha'bi, who heard it from al-Nu'man ibn Bashir, [who said]: I heard the Messenger of Allah, peace and blessings of Allah be upon him—and whenever I heard him say, "I heard the Messenger of Allah, peace and blessings of Allah be upon him," I thought that I would not hear anyone on the pulpit say, "I heard the Messenger of Allah, peace and blessings of Allah be upon him"—saying: "Indeed, there is a piece of flesh in the human being; if it is sound and healthy, the rest of the body is sound and healthy, and if it is diseased, the rest of the body is diseased and corrupted. Behold, it is the heart."