Musnad Ahmad 27:784

Waki' narrated to us, Hammam narrated to us, from Qatadah, from Abu al-Siddiq al-Naji, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you place your dead in their graves, say: 'In the name of Allah, and upon the Sunnah of the Messenger of Allah (peace and blessings of Allah be upon him).'"