Musannaf Ibn Abi Shayba 3:6307

Ibn Idris narrated to us, from ‘Asim ibn Kulayb, from his father, from his maternal uncle, al-Faltan ibn ‘Asim, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "I was shown the Night of Decree (Laylat al-Qadr), then I was made to forget it. So seek it in the last ten nights, on the odd-numbered ones."