Musnad Ahmad 32:564

Hassan narrated to us, Hammad narrated to us, Thabit al-Bunani informed us, from Anas ibn Malik: "The Messenger of Allah (peace and blessings of Allah be upon him) was approached by Gabriel while he was playing with the boys. He took him, laid him down, and split open his chest. He extracted the heart, then split the heart and extracted from it a blood clot. He said: 'This is the portion of Satan in you.' He then washed it in a golden basin with Zamzam water, then joined it together, and returned it to its place. The boys came running to his mother—meaning his wet nurse—and said: 'Muhammad has been killed.' They went to meet him, and he was pale in color." Anas said: "And I used to see the mark of the needle on his chest."