Musannaf Ibn Abi Shayba 37:514

Abu Bakr narrated to us, he said: Al-Fadl ibn Dukayn narrated to us, he said: Yunus ibn Abi Ishaq narrated to us, from Ubayd Allah ibn Bashir ibn Jarir al-Bajali, he said: Ali said: "The last Kharijite group to emerge in Islam will be at al-Rumaylah, the Rumaylah of al-Daskarah. People will go out against them and kill one-third of them, one-third will enter [the city/fold], and one-third will fortify themselves in the monastery, the monastery of Marmar. Among them will be the gray-haired man. People will come to them and bring them down, then they will kill them. That is the last Kharijite group to emerge in Islam."