Musnad Abu Ya'la al-Mawsili 148:68

Ubayd Allah ibn Umar al-Qawariri narrated to us, Fudayl ibn Iyad narrated to us, from Mansur, from Rib'i ibn Hirash, from Abu al-Abyad, from Anas ibn Malik, who said: We used to pray the Asr prayer with the Prophet, may Allah bless him and grant him peace, while the sun was white and high. Then I would go to my clan and find them sitting, so I would say to them: "Stand up and pray, for the Messenger of Allah, may Allah bless him and grant him peace, has already prayed."