Musnad Ahmad 27:1883

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Salim, from his father, who said: The Messenger of Allah (may Allah bless him and grant him peace) was narrating, and he said: "While I was sleeping, I saw myself being 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 my remainder to 'Umar ibn al-Khattab." They said: "How did you interpret that, O Messenger of Allah?" He said: "Knowledge."