Musnad Ahmad 27:1571

Abu al-Yaman narrated to us, Shu'ayb informed us, from al-Zuhri, Salim ibn 'Abd Allah informed me, that 'Abd Allah ibn 'Umar said: I heard the Prophet, may Allah bless him and grant him peace, while he was saying upon the pulpit: "Verily, the tribulation is here"—pointing toward the east—"from where the horn of Satan rises."