Musnad Ahmad 27:1219

Abu al-Nadr narrated to us, Abu Mu'awiyah—meaning Shayban—narrated to us, from Layth, from Mujahid, from Abdullah ibn Umar, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) stood upon al-Safa and al-Marwah." And Umar used to command us to stand upon them from where one could see them.