Musnad Ahmad 32:991

Isma'il narrated to us, Ayyub narrated to us, from Abu Qilabah, 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."