Musnad Ahmad 1012:2

Mu'tamir ibn Sulayman narrated to us, from Humayd, from Anas, from Ubadah ibn al-Samit, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Seek it in the ninth, the seventh, and the fifth," meaning: the Night of Decree (Laylat al-Qadr).