Musnad Ahmad 27:1682

Ya'qub narrated to us, my father narrated to us, from Salih. Ibn Shihab said: Hamzah ibn 'Abdullah ibn 'Umar narrated to me that he heard 'Abdullah ibn 'Umar saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "While I was sleeping, I was brought a cup of milk, and I drank from it until I could see the quenching (of thirst) coming out of my extremities. Then I gave the remainder to 'Umar ibn al-Khattab." Those around him said: "How did you interpret that, O Messenger of Allah?" He said: "Knowledge."