Reciting "assalamu'alaika ayyuhan nabiyyu wa rahmatullahi wa barakaatuh," has been Nasakh (cancelled) by Rasulullah, after he died, Ibn Mas'ud said that after he died, Muslim must recite it "assalamu 'alan nabiyyi wa rahmatullahi wa barakaatuh." This ATSAR of Ibn Mas'ud could be seen in Shifatush Shalatun Nabiyyi Minat Takbiiri Ilat Tasliimi Ka annaka Taraaha by syaikh Nashir Al Albaani Rahimahullah.
It is compulsary offcourse, because that's the act of reciting Salaam to Rasulullah, which will make Allahu Ta'ala to Salaam us.
Assalamu'alaikum wa rahmatullahi wa barakaatuh.