Sahih Bukhari Volume 003, Book 032, Hadith Number 232.

Narated By Ibn 'Umar : Some men amongst the companions of the Prophet were shown in their dreams that the night of Qadr was in the last seven nights of Ramadan. Allah's Apostle said, "It seems that all your dreams agree that (the Night of Qadr) is in the last seven nights, and whoever wants to search for it (i.e. the Night of Qadr) should search in the last seven (nights of Ramadan)."