“He enters into peace; they rest in their beds.” The righteous dead are here spoken of as if they are peacefully sleeping, which in a sense they are; they are sleeping the sleep of death and awaiting their resurrection to everlasting life. While “rest” (or “sleep” ) were well-known idioms for death, calling the grave a “bed” is the figure of speech hypocatastasis, a comparison by implication.
[For more on hypocatastasis, see commentary on Rev. 20:2, “dragon.”]