Sahih Bukhari Volume 9, Book 87, Hadith Number 134.

Narrated By Ibn ‘Umar : I heard Allah’s Apostle saying, “While I was sleeping, I was given a bowl full of milk (in a dream), and I drank of it to my fill until I noticed its wetness coming out of my nails, and then I gave the rest of it to ‘Umar.” They (the people) asked, “What have you interpreted (about the dream)? O Allah’s Apostle?” He said, “(It is Religious) knowledge.”