Musnad Ahmad 32:1238

Ad-Dahhak ibn Makhlad narrated to us, Abdur-Rahman ibn Wardan—a Medinan—narrated to us, he said: We entered upon Anas ibn Malik among a group of the people of Medina. He said: "Have you prayed?"—meaning the Asr prayer. They said: "Yes." We said: "Inform us, may Allah set you right, when did the Messenger of Allah, peace and blessings of Allah be upon him, use to pray this prayer?" He said: "He used to pray it while the sun was white and pure."