Sahih Bukhari Volume 9, Book 87, Hadith Number 157.

Narrated By ‘Abdullah bin ‘Umar : I heard Allah’s Apostle saying, “While I was sleeping, I saw that a cup full of milk was brought to me and I drank of it and gave the remaining of it to ‘Umar bin Al-Khattab.” They asked. What have you interpreted (about the dream)? O Allah’s Apostle?” The Prophet said. “(It is Religious) knowledge.”