Musnad Ahmad 1012:97

Zakariya ibn 'Adi narrated to us, 'Ubayd Allah ibn 'Amr informed us, from 'Abd Allah ibn Muhammad ibn 'Aqil, from 'Umar ibn 'Abd al-Rahman, from 'Ubadah ibn al-Samit, who said: The Messenger of Allah (may Allah bless him and grant him peace) informed us about the Night of Decree (Laylat al-Qadr) and said: "It is in the month of Ramadan, so seek it in the last ten nights; for it is on an odd night: the twenty-first night, or the twenty-third, or the twenty-fifth, or the twenty-seventh, or the twenty-ninth, or the last night of Ramadan. Whoever stands in it (in prayer) out of seeking reward, his past sins will be forgiven."