Leveling the graves is from the Sunnah. The Jews and Christians have raised their graves, so do not imitate them.
إن تسوية القبور من السنة، وقد رفعت اليهود والنصارى فلا تشبهوا بهما.
No references are available.
Tabarani related it in Al-Kabir (19/352) from Husayn bin Ishaq Tustari, who said, Wahb bin Baqiya told us, Khalid bin Abdullah informed us, from Imran bin Hudayr, from Mujallaz, that Muawiya said, then he mentioned it.Haythami said in Al-Majmu (3/352), "Its narrators are the narrators of the sahih."I say, it is as he said.