Sahih al-Bukhari Book 32, Hadith 8
The Prophet (ﷺ) said, "Look for the Night of Qadr in the last ten nights of Ramadan ,' on the night when
nine or seven or five nights remain out of the last ten nights of Ramadan (i.e. 21, 23, 25, respectively).
Allah's Messenger (ﷺ) used to practice I`tikaf in the last ten nights of Ramadan and used to say, "Look for the Night of Qadr in the last ten nights of the month of Ramadan."
Allah's Messenger (ﷺ) 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).