Sahih Muslim Book 31, Hadith Number 5888.
Chapter : The excellent qualities of ‘Umar (Allah be pleased with him).
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 being 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: How do you interpret it? He said: This implies knowledge.