Musannaf Ibn Abi Shayba 3:6285

Waki’ narrated to us, he said: Sufyan narrated to us, from ‘Abdullah ibn Dinar, from Ibn ‘Umar, he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Seek the Night of Decree (Laylat al-Qadr) in the last ten [nights] of the month of Ramadan."