Musannaf Ibn Abi Shayba 3:6238

Yazid ibn Harun narrated to us, from Abd al-Malik, from Ata, from Ubayd ibn Umayr, that he used to say: "Guard the prayers and the middle prayer," and the prayer—(the prayer)—of Asr. He said: And Ata used to hold the view that the middle prayer is the morning prayer (Salat al-Ghadah).