Musnad Abu Ya'la al-Mawsili 125:7

Abu Bakr ibn Abi Shaybah narrated to us, Ibn 'Uyaynah narrated to us, from Ibrahim ibn Maysarah, who heard Anas ibn Malik saying: The Messenger of Allah (may Allah bless him and grant him peace) prayed Dhuhr in Medina as four rak'ahs. And at Dhu'l-Hulayfah, he prayed two rak'ahs, meaning 'Asr.