Musnad Ahmad 27:360

Yazid ibn Harun narrated to us, Shu'bah informed us, from Abdullah ibn Dinar, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever is seeking it, let him seek it on the night of the twenty-seventh." And he said: "Seek it on the night of the twenty-seventh," meaning the Night of Decree (Laylat al-Qadr).