Musnad Ahmad 1156:85

Ali ibn Ishaq narrated to us, Abdullah narrated to us, Ibn Lahi'ah informed us, Yazid ibn Abi Habib narrated to me, from Na'im, the freed slave of Umm Salamah: "That the Prophet, peace and blessings of Allah be upon him, forbade that a grave be plastered with gypsum, that a structure be built over it, or that one should sit upon it." My father said: "Umm Salamah is not in it [the chain of narration]."