Musnad Ahmad 2:217

Husayn ibn Ali narrated to us, from Za'idah, from 'Asim, from his father, from Ibn 'Abbas, who said: 'Umar said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever among you is seeking the Night of Decree, let him seek it in the last ten [nights], on the odd [numbered] ones."