Musnad Ahmad 830:32

Yahya narrated to us, from 'Uyaynah, who said: My father narrated to me, saying: Laylat al-Qadr was mentioned in the presence of Abu Bakrah, and he said: "I will not seek it except in the last ten nights, after something I heard from the Messenger of Allah, peace and blessings of Allah be upon him. I heard him saying: 'Seek it in the last ten nights: when nine remain, or seven remain, or five remain, or three remain, or the last night.'"