Musnad Abu Ya'la al-Mawsili 120:2

Salih ibn Malik narrated to us, Abd al-Aziz ibn Abd Allah ibn Abi Salamah narrated to us, from Muhammad ibn al-Munkadir, who said: I heard Anas ibn Malik saying: 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] while he was traveling to Mecca.