Musnad Abu Ya'la al-Mawsili 113:22

Ishaq narrated to us, Hammad bin Zayd narrated to us, from Ayyub, 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].