Sahih Bukhari
Praying at Night in Ramadaan (Taraweeh)
Volume: 003 / Book Number: 032 / Hadith Number: 232
Narrated 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)."