ANSWER:

Alaihis salam- upon him be salaam and this is usually used with prophets and angels etc. 

Salamun Alaihi- salam be upon him.

RadhiAllahu anhu- May Allah be pleased with him, and this is usually used for companions. 

What is the difference between Alayhi salam(as), Salamun alayha(sa) and Radhi Allahu anhu(ra)?

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