Musannaf Ibn Abi Shayba 3:4902

Al-Fadl ibn Dukayn narrated to us, from Bukayr ibn Amir, from Abd al-Rahman ibn al-Aswad, who said: "Whoever prays four rak'ahs after the final Isha [prayer] is equivalent to the like of them from the Night of Decree (Laylat al-Qadr)."