Musnad Abu Ya'la al-Mawsili 2:26

Muhammad ibn Abdullah ibn Numayr narrated to us, Ibn Fudayl narrated to us, from Asim, from his father, from Ibn Abbas, from Umar, who 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 ten nights of Ramadan."