Musannaf Ibn Abi Shayba 3:961

Ibn al-Mubarak narrated to us, from Ma’mar, from ‘Abdullah ibn ‘Uthman, from Ibn Lutaybah, who said: Abu Hurayrah said: Perform the ‘Isha prayer when the twilight has disappeared and the night has darkened, between you and the third of the night. And whatever you hasten [to perform] after the disappearance of the whiteness of the horizon, it is better.