Where does “سلام” come from?

سلام (Classical Persian) comes from Arabic سَلَام, from Proto-Semitic šalām- — peace, well-being, welfare.

Ancestry of “سلام”, step by step

Step	Language	Word	Meaning
1	Arabic	سَلَام	peace; hello; greeting; peace
2	Proto-Semitic	šalām-	peace, well-being, welfare