Sahih Muslim
The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fadail)
Book Number: 030 / Hadith Number: 5792
As-Sa'ib b. Yazid reported: My mother's sister took me to Allah's Messenger (may peace be upon him) and said: Allah's Messenger, here is the son of my sister and he is ailing. He touched my head and invoked blessings upon me. He then performed ablution and I drank the water left from his ablution; then I stood behind him and I saw the seal between his shoulders.