Musnad Ahmad 32:969

'Abd al-Rahman narrated to us, Sufyan narrated to us, from Mansur, from Rib'i, from Abu al-Abyad, from Anas ibn Malik, who said: "The Prophet, peace and blessings of Allah be upon him, used to pray the 'Asr prayer while the sun was white and high. Then I would return to my family and my clan in the outskirts of Medina, and I would say: 'The Messenger of Allah, peace and blessings of Allah be upon him, has prayed, so stand up and pray.'"