Musannaf Ibn Abi Shayba 4:673

Ibn ‘Ulayyah narrated to us, from al-Dastawa’i, from Yahya ibn Abi Kathir, from Abu Salamah, from Abu Sa’id al-Khudri, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "I was shown the Night of Decree (Laylat al-Qadr), then I was made to forget it, or I forgot it. So seek it in the last ten nights, in the odd-numbered ones."