Musnad Ahmad 1153:1672

Waki' narrated to us, Hisham and Ibn Numayr narrated to us, from Hisham, from his father, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Seek the Night of Decree (Laylat al-Qadr) in the last ten nights of Ramadan."