Sahih al-Bukhari Book 32, Hadith 7

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."

Previous Hadith

The Prophet (ﷺ) said, "Look for (the Night of Qadr).

Sahih al-Bukhari Book 32, Hadith 6
Next Hadith

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).

Sahih al-Bukhari Book 32, Hadith 8