Musnad Ahmad 32:1329

Hammad ibn Khalid narrated to us, from Ibn Abi Dhi’b, from Ibn Shihab, from Anas, 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 bright. Then, a person would go to the Al-Awali (the outskirts of Medina) and arrive there while the sun was still high."