Musannaf Ibn Abi Shayba 3:5739

Waki' narrated to us, he said: Sufyan narrated to us, from Ibn al-Munkadir and Ibrahim ibn Maysarah, that they heard Anas saying: I prayed the Dhuhr prayer with the Prophet, peace and blessings of Allah be upon him, in Medina as four [rak'ahs], and the Asr prayer at Dhu al-Hulayfah as two [rak'ahs].