Musnad Ahmad 27:489

Muhammad ibn Bakr narrated to us, Ibn Jurayj informed us, al-Zuhri narrated to me, from the hadith of Salim ibn Abdullah, that Abdullah ibn Umar said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Seek the Night of Decree (Laylat al-Qadr) in the last seven [nights] of the month of Ramadan."