Musnad Ahmad 33:320

Yahya narrated to us, from Ibn Jurayj, Abu al-Zubayr informed me, he said: I heard Jabir ibn Abdullah saying: "I saw the Messenger of Allah (peace and blessings of Allah be upon him) throwing the pebbles on the Day of Sacrifice in the forenoon, alone. As for after that, it was after the sun had passed its zenith."