Musnad Abu Ya'la al-Mawsili 120:3

Muhammad ibn al-Khattab narrated to us, Mu'ammal narrated to us, Sufyan narrated to us, from Muhammad ibn al-Munkadir, from Anas, who said: I prayed the Dhuhr prayer with the Prophet, peace and blessings of Allah be upon him, in Medina as four units. And he prayed the Asr prayer at Dhu al-Hulayfah as two units.