Musnad Abu Ya'la al-Mawsili 118:58

Zuhayr narrated to us, Hashim ibn al-Qasim narrated to us, al-Layth narrated to us, Ibn Shihab narrated to us, from Anas ibn Malik, that he informed him: That the Messenger of Allah (may Allah bless him and grant him peace) used to pray the Asr prayer while the sun was high and bright. Then the goer would go to the high grounds (al-Awali) while the sun was still high.