Musnad Ahmad 5:547

'Abdullah narrated to us, Suwayd bin Sa'id narrated to me, 'Abd al-Hamid bin al-Hasan al-Hilali informed me, from Abu Ishaq, from Hubayrah bin Yarim, from 'Ali: That the Messenger of Allah (ﷺ) said: "Seek the Night of Decree (Laylat al-Qadr) in the last ten nights of Ramadan. If you are overcome, then do not be overcome regarding the remaining seven."