Musnad Ahmad 27:834

'Abd al-Rahman narrated to us, from Sufyan, from 'Abd Allah ibn Dinar, [who said]: I heard Ibn 'Umar saying: The Messenger of Allah, peace and blessings of Allah be upon him, was asked about the Night of Decree (Laylat al-Qadr), and he said: "Seek it in the last seven [nights]."