Musnad Ahmad 27:11

Hashim narrated to us, Yahya bin Sa'id informed us, from 'Abdullah bin Abi Salamah, from Ibn 'Umar, who said: "We set out early with the Messenger of Allah, peace and blessings of Allah be upon him, to 'Arafat. Among us were those who were proclaiming the greatness of Allah (Takbir), and among us were those who were proclaiming the oneness of Allah (Talbiyah)."