Sahih Bukhari Volume 002, Book 021, Hadith Number 255.
Narated By Nafi : Ibn 'Umar said, "In the life-time of the Prophet I dreamt that a piece of silk cloth was in my hand and it flew with me to whichever part of Paradise I wanted. I also saw as if two persons (i.e. angels) came to me and wanted to take me to Hell. Then an angel met us and told me not to be afraid. He then told them to leave me. Hafsa narrated one of my dreams to the Prophet and the Prophet said, "Abdullah is a good man. Would that he offer the night prayer (Tahajjud)!" So after that day 'Abdullah (bin 'Umar) started offering Tahajjud. The companions of the Prophet (p.b.u.h) used to tell him their dreams that (Laila-tul-Qadr) was on the 27th of the month of Ramadan. The Prophet said, "I see that your dreams agree on the last ten nights of Ramadan and so whoever is in search of it should seek it in the last ten nights of Ramadan."