Musnad Abu Ya'la al-Mawsili 113:21

Abu Khaythama narrated to us, Isma'il narrated to us, Ayyub narrated to us, from Abu Qilabah, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) prayed Dhuhr in Medina as four [rak'ahs], and he prayed 'Asr at Dhu al-Hulayfah as two [rak'ahs].