Musnad Ahmad 1132:1

Ali ibn Abdullah narrated to us, Ibn Wahb narrated to us, Amr ibn al-Harith informed me, from Bakr ibn Sawadah al-Judhami, from Ziyad ibn Nu'aym al-Hadrami, from Amr ibn Hazm, who said: The Messenger of Allah, peace and blessings of Allah be upon him, saw me leaning against a grave, so he said: "Do not harm the occupant of this grave—or do not harm him."