Musnad Ahmad 27:1966

Wahb ibn Jarir narrated to us, my father narrated to us, I heard Yunus, from al-Zuhri, from Hamzah ibn Abdullah ibn Umar, from his father, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "I was brought, while I was sleeping, a cup of milk, and I drank from it until I saw the milk coming out from my fingernails. Then I gave the remainder to Umar ibn al-Khattab." He (Umar) said: "O Messenger of Allah, how did you interpret it?" He said: "Knowledge."