Musannaf Ibn Abi Shayba 3:6429

Ibn al-Mubarak narrated to us, from Ma'mar, from 'Abdullah ibn 'Uthman ibn Khuthaym, from Ibn al-Lutbiyyah, who said: Abu Hurayrah said to me: "Pray the 'Isha' [prayer] when the twilight has departed and the darkness of the night has set in, between you and the [end of the] first third of the night. And whatever you hasten [to perform] after the disappearance of the whiteness of the horizon, it is better."