Musnad Ahmad 1012:48

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, saying: Sa’id ibn Salamah—meaning Ibn Abi al-Husam—narrated to us, saying: ‘Abdullah ibn Muhammad ibn ‘Aqil narrated to us, from ‘Umar ibn ‘Abd al-Rahman, from ‘Ubadah ibn al-Samit, that he asked the Messenger of Allah (may Allah bless him and grant him peace) about the Night of Decree (Laylat al-Qadr). The Messenger of Allah (may Allah bless him and grant him peace) 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 prayer during it, seeking it out of faith and anticipation of reward, and then it is granted to him, his past and future sins will be forgiven."