Musnad Ahmad 32:1058

Bahz narrated to us, Jarir ibn Hazim narrated to us, he said: I heard Qatadah narrating from Anas ibn Malik, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to be cupped three times: one on his upper back (kahil), and two on the sides of his neck (akhda'ayn)."