Musnad Ahmad 32:1490

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, from Mansur, from Rib'i, Abu al-Abyad narrated to us, Anas ibn Malik narrated to us, he said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray the 'Asr prayer with us while the sun was white and high. Then I would return to my people while they were in a part of Medina, and I would find them sitting. I would say to them: 'Stand up and pray, for the Messenger of Allah (peace and blessings of Allah be upon him) has already prayed.'"