Charles Ellicott Commentary Hebrews 4:7

"he again defineth a certain day, To-day, saying in David so long a time afterward (even as hath been said before), To-day if ye shall hear his voice, Harden not your hearts." — Hebrews 4:7 (ASV)

Again, He limits.—Better, He again marks out (or, defines). The next step taken (see the last Note) is to point out that, long after the occupation of Canaan, the Psalmist—God speaking in the Psalm—says “To-day,” in pleading with Israel. The implied meaning is as if He said, “Do not harden your hearts today, lest I swear to you also, You shall not enter into My rest.”

In David.—Probably this is equivalent to saying, In the Book of Psalms. In the Septuagint, however,Psalms 95:0 is ascribed to David.

After so long a time.—The period intervening between the divine sentence on the rebels in the wilderness (Numbers 14:0) and the time of the Psalmist.

As it is said.—The best manuscripts read, as it has been said before.