Musnad Ahmad 27:1651

'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 (may Allah bless him and grant him peace) said: "When you place your dead in the graves, say: 'In the name of Allah, and upon the Sunnah of the Messenger of Allah (may Allah bless him and grant him peace).'"