Musnad Ahmad 1131:2

Hassan narrated to us, Ibn Lahi'ah narrated to us, Bakr ibn Suwadah narrated to us, from Ziyad ibn Nu'aym al-Hadrami, from 'Umarah ibn Hazm, who said: The Messenger of Allah (peace and blessings of Allah be upon him) saw me sitting on a grave. And in another place, Ziyad ibn Nu'aym said that Ibn Hazm—either 'Amr or 'Umarah—said: The Messenger of Allah (peace and blessings of Allah be upon him) saw me while I was leaning on a grave, and he said: "Get down from the grave; do not harm the occupant of the grave, and let him not harm you."