Sahih Muslim Book 30, Hadith Number 5793.

Chapter : The fact pertaining to the seal of his prophethood, its characteristic feature and its location on his body.

Abdullah b. Sarjis reported: I saw Allah’s Apostle (may peace be upon him) and ate with him bread and meat, or he said Tharid (bread soaked in soup). I said to him: Did Allah’s Apostle (may peace be upon him) seek forgiveness for you? He said: Yes, and for you, and he then recited this verse: “Ask forgiveness for thy sin and for the believing men and believing women” (xlvii. 19). I then went after him and saw the Seal of Prophethood between his shoulders on the left side of his shoulder having spots on it like moles.