Musannaf Ibn Abi Shayba 3:3571

Waki’ narrated to us, from Bishr, from a sheikh of the Ansar, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever prays four [rak'ahs] before the Dhuhr [prayer], it is for him like the freeing of a slave from the children of Ishmael."