Musannaf Ibn Abi Shayba 37:209

Abu Khalid al-Ahmar narrated to us, from Sa'id ibn 'Abd al-'Aziz, from Makhul, who said: 'Umar said: "O people, emigrate before the Abyssinians emerge from the valleys of Banu 'Ali—a fire that will come from the direction of Yemen. It will gather the people; it will move when they move, and it will stay when they stay, to the extent that it will even gather the dung beetles, until it ends with them at Busra. And a man will fall, so he will stand still until it overtakes him."