Sahih Bukhari Volume 008, Book 075, Hadith Number 363.

Narated By As-Sa'ib bin Yazid : My aunt took me to Allah's Apostle and said, "O Allah's Apostle! My sister's son is sick." So he passed his hand over my head and invoked for Allah's blessing upon me and then performed the ablution. I drank from the water of his ablution and I stood behind him and looked at his Khatam (the seal of Prophet hood) between his shoulders (and its size was) like the button of a tent.