It is narrated from Sayyiduna Wathilah bin al-Asqa' (may Allah be pleased with him) that when the deceased was placed in his grave, the Messenger of Allah (peace and blessings be upon him) would say: "Bismillah, and according to the Sunnah of the Messenger of Allah (peace and blessings be upon him)." And he (peace and blessings be upon him) would place a clod of earth behind the nape of his neck, a clod between his shoulders, and between his knees, and another behind him.
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18371