Musnad Ahmad 830:45

Yazid ibn Harun narrated to us, 'Uyaynah ibn Abd al-Rahman informed us, from his father, who said: Laylat al-Qadr was mentioned in the presence of Abu Bakrah. He said: "I will not seek it after what I heard from the Messenger of Allah (ﷺ) except in the last ten nights. I heard the Messenger of Allah (ﷺ) say: 'Seek it in the last ten nights, in the odd-numbered ones among them.'" He said: Abu Bakrah used to pray during the twenty [nights] of Ramadan as he prayed during the rest of the year, but when the [last] ten entered, he would exert himself.