Musnad Ahmad 32:391

Muhammad ibn Ja'far and Hajjaj narrated to us, they said: Shu'bah narrated to us, from Mansur, from Rib'i ibn Hirash, from Abu al-Abyad—Hajjaj said: "A man from Banu 'Amir"—from Anas ibn Malik, who said: "The Messenger of Allah, may Allah bless him and grant him peace, used to pray the 'Asr prayer while the sun was white and high."