Sahih Muslim
The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fadail Al-Sahabah)
Book Number: 031 / Hadith Number: 5888
Hamza b. Abdullah b. 'Umar b. Khattab reported on the authority of his father that Allah's Messenger (may peace be upon him) said: While I was asleep I saw (in a dream) a cup containing milk bein. presented to me. I took out of that until I perceived freshness being reflected through my nails. Then I presented the leftover to 'Umar b. Khattab. They said: Allah's Messenger: Fow do you interpret it? He said: This implies knowledge.