Musnad Ahmad 32:139

Sufyan narrated to us, he said: I heard Ibrahim ibn Maysarah, and Muhammad ibn al-Munkadir narrated to us, I heard them both saying: We heard Anas saying: "I prayed with the Prophet, peace and blessings of Allah be upon him, four [rak'ahs] in Medina, and two [rak'ahs] at Dhu al-Hulayfah."