You never say salam to graves when you come across a graveyard?
When Nabi Kareem Sallallahu Alaihi passed near graveyard of Madina Munawwarah then he said:
“Assalam O Aalaikum Ya Ahlul Quboor Yagfirullahu Lana Walakum wa antum salfuna wa nahnu bil asari” (Mishkaat - Baab Ziaratul Quboor, Tirmizi)