Musnad Ahmad 10:29

Abu al-Ala al-Hasan ibn Suwar narrated to us, Hisham ibn Sa’d narrated to us, from al-Zuhri, from Humayd ibn Abd al-Rahman ibn Awf, from Abd al-Rahman ibn Awf, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "If you hear of it in a land, and you are not in it, then do not enter it. And if it occurs while you are in it, then do not exit it, fleeing from it."