Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
It is not allowed, for it is a fast way for people to start thinking holy of such people. The prophet (peace be upon him) spoke about how people of the Book prior to us did that and ended up worshipping such people. The case of the prophet’s grave is different, for the expansion of the Mosque happened after his death (peace be upon him).

 

Almighty Allah knows best.