Musannaf Ibn Abi Shayba 3:6284

Waki’ narrated to us, he said: Ibn ‘Uyaynah narrated to us, from ‘Abd al-Rahman, from his father, from Abu Bakrah, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Seek the Night of Decree (Laylat al-Qadr) in the last ten nights of the month of Ramadan, when nine nights remain, or seven nights remain, or five nights remain, or three, or the last night."