Musnad Ahmad 32:1292

'Abd al-Malik ibn 'Amr narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, from Anas ibn Malik: "The Prophet, peace and blessings of Allah be upon him, used to pray the 'Asr prayer while the sun was white and bright. Then, the one going to the 'Awali would depart and reach them while the sun was still high."