Musnad Ahmad 1156:84

Hassan narrated to us, Ibn Lahi'ah narrated to us, Yazid ibn Abi Habib narrated to us, from Na'im, the freed slave of Umm Salamah, from Umm Salamah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, forbade that one should build upon a grave, or that it should be plastered."