Musnad Ahmad 1683

It was narrated from "Abdullah bin ‘Abbas that "Umar bin al-Khattab set out for Syria, then when he was in Sargh, he was met by the commanders of the troops, Abu ‘Ubaidah bin al-Jarrah and his companions, and they told him that an epidemic had broken out in Syria.... and he narrated the same Hadeeth. He said: ‘Abdur-Rahman bin "Awf came, who had been away on an errand, and said: I know something about this. I heard the Messenger of Allah (ﷺ) say: "If it [the epidemic] is in a land and you are there, do not leave it, fleeing from it; and if you hear of it in some land, do not go there.