Musnad Ahmad 32:783

Hajjaj narrated to us, Shu'bah narrated to me, from Mansur, from Rib'i ibn Hirash, from Abu al-Abyad, a man from Banu 'Amir, from Anas ibn Malik, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray the 'Asr prayer while the sun was white and high."