Musnad Ahmad 27:1416

Qutaybah ibn Sa'id narrated to us, Layth ibn Sa'd narrated to us, from 'Uqayl, from al-Zuhri, from Hamzah ibn 'Abd Allah, from 'Abd Allah ibn 'Umar, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) say: "While I was sleeping, I was brought a cup of milk, and I drank from it. Then, I gave my remainder to 'Umar ibn al-Khattab." They said: "How did you interpret it, O Messenger of Allah?" He said: "Knowledge."