What is the ruling on saying "peace be upon him" for a Companion?

The use of the phrase "peace be upon him" (عليه السلام) for individuals other than prophets is a matter of scholarly disagreement.

Many scholars lean towards its permissibility if used occasionally and not adopted as a regular slogan. They hold that there is no harm in invoking prayers or peace upon a single Companion occasionally, provided that this is not adopted as a slogan singling out a particular Companion over someone who is superior to them.

They cite as evidence the Almighty's saying: "It is He who confers blessing upon you, and His angels" (33:43), and His saying: "Those are the ones upon whom are blessings from their Lord and mercy" (2:157), and His saying: "And invoke blessings upon them; indeed, your invocations of blessings are a comfort for them" (9:103). They also cite the action of the Prophet, peace and blessings be upon him, when he invoked blessings upon those who paid zakat and upon the family of Abu Awfa.

Other scholars hold the view that it is not permissible to single out individuals other than prophets with prayers or peace, because this has become a distinctive mark for the prophets, just as the phrase "Glorified and Exalted be He" (عز وجل) is specific to Allah, the Almighty.

Those who prohibit it differ between outright prohibition, detestable dislike (karahah tanzihiyyah), or being contrary to what is better. The most preponderant view is that it is disliked with detestable dislike (karahah tanzihiyyah) because it has become a mark of the people of innovation.