Musnad Ahmad 464:1

Al-Walid ibn Muslim narrated to us, he said: I heard Ibn Jabir saying: Busr ibn Ubayd Allah al-Hadrami narrated to me that he heard Wathilah ibn al-Asqa’, a companion of the Messenger of Allah, may Allah bless him and grant him peace, saying: Abu Marthad al-Ghanawi narrated to me, he heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Do not pray facing the graves, and do not sit upon them."