Musnad Ahmad 27:994

'Affan 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: "Whoever is seeking it, let him seek it in the last ten [nights]. If he is unable or weak, then let him not be overcome regarding the remaining seven."