Musnad Ahmad 27:364

Yazid narrated to us, Hammam ibn Yahya informed us, from Qatadah, from Abu al-Siddiq al-Naji, from Ibn Umar, from the Prophet (peace and blessings of Allah be upon him) who said: "When you place your dead in the grave, say: 'In the name of Allah, and upon the creed of the Messenger of Allah (peace and blessings of Allah be upon him).'"