Musnad Ahmad 32:875

'Abd al-Rahman narrated to us, from Sufyan, from Muhammad ibn al-Munkadir and Ibrahim ibn Maysarah, from Anas ibn Malik: "The Messenger of Allah (peace and blessings of Allah be upon him) prayed Dhuhr in Medina as four rak'ahs, and he prayed 'Asr at Dhu al-Hulayfah as two rak'ahs."