Sahih Bukhari Volume 7, Book 70, Hadith Number 574.

Narrated By As-Sa’ib : My aunt took me to Allah’s Apostle and said, “O Allah’s Apostle! My nephew is- ill.” The Prophet touched my head with his hand and invoked Allah to bless me. He then performed ablution and I drank of the remaining water of his ablution and then stood behind his back and saw “Khatam An-Nubuwwa” (The Seal of Prophet hood) between his shoulders like a button of a tent.