Musnad Abu Ya'la al-Mawsili 120:1

Abu Khaythama Zuhayr ibn Harb narrated to us, Sufyan ibn 'Uyayna narrated to us, from Ibn al-Munkadir and Ibrahim ibn Maysara, that they heard Anas ibn Malik saying: I prayed the Dhuhr prayer with the Prophet, peace and blessings of Allah be upon him, in Medina as four [rak'ahs], and the 'Asr prayer at Dhu al-Hulayfah as two [rak'ahs].