Musnad Ahmad 27:1036

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Uqbah ibn Hurayth, I heard Ibn 'Umar saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Seek it in the last ten [nights]—meaning the Night of Decree (Laylat al-Qadr). If one of you is weak or unable, then let him not be overcome regarding the remaining seven."