Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1376
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).