Musnad Abu Ya'la al-Mawsili 61:1

Al-Abbas ibn al-Walid al-Narsi narrated to us, Abdullah ibn al-Mubarak narrated to us, Abd al-Rahman ibn Yazid ibn Jabir narrated to me, he said: I heard Busr ibn Ubayd Allah—and he said once: from Busr ibn Ubayd Allah—from Abu Idris al-Khawlani, from Wathilah ibn al-Asqa, from Abu Marthad al-Ghanawi, he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Do not sit upon the graves, and do not pray towards them."