Musnad Ahmad 27:531

Ishaq ibn Sulayman narrated to us, I heard Hanzalah, I heard Salim saying: I heard Abdullah ibn Umar saying: I saw the Messenger of Allah, may Allah bless him and grant him peace, pointing toward the east, or he said: Indeed, the Messenger of Allah, may Allah bless him and grant him peace, was pointing toward the east, saying: "Lo, indeed the tribulation is here. Lo, indeed the tribulation is here. Lo, indeed the tribulation is here, from where the horns of Satan rise."