Musnad Ahmad 32:1544

Ya'qub narrated to us, my father narrated to us, from Muhammad ibn Ishaq, who said: Muhammad ibn al-Munkadir al-Taymi narrated to me, from Anas ibn Malik al-Ansari, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, led us in the Dhuhr prayer in his mosque in Medina, [performing] four rak'ahs. Then he led us in the 'Asr prayer at Dhu al-Hulayfah, [performing] two rak'ahs, being secure and not fearing, during the Farewell Pilgrimage."