Musnad Abu Ya'la al-Mawsili 154:125

Abu Khaythama narrated to us, Abd al-Samad narrated to us, Hammam narrated to us, Qatada narrated to us, from Abu al-Siddiq, from Ibn Umar, that the Messenger of Allah (may Allah bless him and grant him peace) said: "When you place your dead in the lahd (niche in the side of the grave), say: 'In the name of Allah, and upon the Sunnah of the Messenger of Allah.'"