Sahih Bukhari Volume 3, Book 32, Hadith Number 239.

Narated By Ibn ‘Abbas : Allah’s Apostle said, “The Night of Qadr is in the last ten nights of the month (Ramadan), either on the first nine or in the last (remaining) seven nights (of Ramadan).” Ibn ‘Abbas added, “Search for it on the twenty-fourth (of Ramadan).”