Musannaf Ibn Abi Shayba 1:79

79. Kathir ibn Hisham narrated to us, from Ja'far ibn Burqan, who said: I asked al-Zuhri, "How much is sufficient for the wudu regarding the face and the forearms?" He said, "I do not see anything but one thorough [washing] as being sufficient." He said: So I said to him, "Indeed, Maymun says: three upon the face and three upon the forearms." He said, "That is the most complete wudu."