Musnad Ahmad 27:981

'Affan narrated to us, 'Abd al-'Aziz bin Muslim narrated to us, 'Abdullah bin Dinar narrated to us, from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about the Night of Decree (Laylat al-Qadr). He said: "Seek it in the last seven [nights]."