Sahih Bukhari Volume 5, Book 57, Hadith Number 30.

Narated By Hamza’s father : Allah’s Apostle said, “While I was sleeping, I saw myself drinking (i.e. milk), and I was so contented that I saw the milk flowing through my nails. Then I gave (the milk) to ‘Umar.” They (i.e. the companions of the Prophet) asked, “What do you interpret it?” He said, “Knowledge.”