Musnad Ahmad 1012:76

'Abd al-Malik ibn 'Amr narrated to us, Zuhayr ibn Muhammad narrated to us, from 'Abd Allah ibn Muhammad—meaning Ibn 'Aqil—from 'Umar ibn 'Abd al-Rahman, from 'Ubadah ibn al-Samit, that he said: "O Messenger of Allah, inform us about the Night of Decree (Laylat al-Qadr)." The Messenger of Allah, peace and blessings of Allah be upon him, said: "It is in Ramadan. Seek it in the last ten nights, for it is on an odd night: on the twenty-first, or the twenty-third, or the twenty-fifth, or the twenty-seventh, or the twenty-ninth, or on the last night. Whoever stands in it (in prayer) out of faith and seeking reward, his past and future sins will be forgiven."