From Sahih Bukhari (Hadith 2021):
Narrated Ibn `Abbas: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)
Reference: Bukhari 2021