Albert Barnes Commentary Hebrews 4:5

"and in this [place] again, They shall not enter into my rest." — Hebrews 4:5 (ASV)

And in this place again (Psalms 95:11).

If they shall enter—that is, they shall not enter. See the notes on Hebrews 3:11.

The object of quoting this here seems to be twofold:

1. To show that even in this Psalm God spoke of his rest, and said that they should not enter into it; and
2. It is connected with Hebrews 4:6 and is designed to show that it was implied that a rest yet remained. "That which deserves to be called the divine rest is spoken of in the Scriptures; and as they did not enter into it, it follows that it must be in reserve for some others, and that the promise must still remain."