Musannaf Ibn Abi Shayba 3:5738

Ibn ‘Uyaynah narrated to us, from Ibrahim ibn Maysarah, that he heard Anas saying: The Messenger of Allah (peace and blessings of Allah be upon him) prayed Dhuhr in Medina as four [rak'ahs], and at Dhu al-Hulayfah as two [rak'ahs], meaning 'Asr.