Musnad Ahmad 27:582

Bahz narrated to us, Shu'bah narrated to us, 'Uqbah ibn Hurayth narrated to us, I heard 'Abdullah ibn 'Umar say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever is seeking it, let him seek it in the ten [last nights of Ramadan]. If he is unable or weak, then let him not be overcome regarding the remaining seven."