Musnad Ahmad 464:2

'Attab ibn Ziyad narrated to us, he said: 'Abdullah—meaning Ibn al-Mubarak—narrated to us. My father said: And 'Ali ibn Ishaq narrated to us, he said: 'Abdullah narrated to us, he said: 'Abdur-Rahman ibn Yazid ibn Jabir narrated to us, and he said: Busr ibn 'Ubaydullah narrated to us. 'Ali said: Busr ibn 'Ubaydullah narrated to me, he said: I heard Abu Idris saying: I heard Wathilah ibn al-Asqa' saying: I heard Abu Marthad al-Ghanawi saying: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Do not sit upon the graves, and do not pray towards them."