Musnad Ahmad 25:684

'Affan narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, 'Asim al-Ahwal narrated to us, from Lahiq ibn Humayd and 'Ikrimah, they both said: 'Umar said: "Who knows when the Night of Decree (Laylat al-Qadr) is?" They said: So Ibn 'Abbas said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is in the ten [last nights], in seven that have passed, or seven that remain."