Musannaf Ibn Abi Shayba 3:6286

Abu Bakr narrated to us, he said: Ali bin Mushir narrated to us, from al-Shaybani, from Jabalah and Muharib, from Ibn Umar, he 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 the month of Ramadan."