When refering to the sahaba we say “RadhiyAllahu ‘anhu/a”, when refering to the taabi’ee scholars we say “Rahimahullaah”, we also say “Rahmathullaahi ‘alaih” for the Imaams, then for the contemporary scholars alive, we say “Hafidahullaah” etc. Correct me if I am wrong or incomplete. Shaikh, how are we deciding or agreeing upon these different salutations? Is it a formal process like issuing a Fatwa, or otherwise?
Radia Allahu anhu, is used with the companions because it was mentined in the Quran that Allah was pleased with them and they were pleased with Him.
Rahimahu Allah is used with everyone else even with us as this is supplication that Allah may have mercy on him.