Abu Dawud Book 2, Hadith Number 1379.
Chapter : The tradition that seventeenth night of ramadhan is lailat alqadr.
Narated By Abdullah ibn Mas’ud : The Apostle of Allah (PBUH) said to us: Seek it (laylat al-Qadr) on the seventeenth night of Ramadan, and on the twenty first night, and on the twenty-third night. He then kept silence.