Abu Dawud Book 2, Hadith Number 1376.
Chapter : Lailat alqadr.
Narated By Ibn Abbas : The Prophet (PBUH) said: Seek lailat al-qadr in the last ten nights of Ramadhan: when nine (nights) remain (i.e. on the twenty-first), when seven (nights) remain (i.e. on the twenty-third), and when five (nights) remain (i.e. on the twenty fifth).