Sahih Bukhari Book 24. Obligatory Charity Tax (Zakat)
Sahih Bukhari Volume 003, Book 032, Hadith Number 238.
Narated By Ibn Abbas : The Prophet 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)."