Musnad Ahmad 27:1201

Abu al-Nadr Hashim ibn al-Qasim narrated to us, Shu'bah narrated to us, from 'Uqbah ibn Hurayth, I heard Ibn 'Umar narrating from the Prophet, may Allah bless him and grant him peace, who said: "Whoever among you is seeking [Laylat al-Qadr], let him seek it in the last ten [nights]. And if one of you is weak or overcome, let him not be overcome regarding the remaining seven."