Musnad Ahmad 27:921

'Affan narrated to us, Hammam narrated to us, Qatadah narrated to us, from Abu al-Siddiq, from Ibn 'Umar—Hammam said: "In my book"—he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "When you place your dead in the grave, say: 'In the name of Allah, and upon the Sunnah of the Messenger of Allah, peace and blessings of Allah be upon him.'"