The wilderness warning in Hebrews
Hebrews quotes Psalm 95's warning from Israel's wilderness years and urges its readers not to harden their hearts in unbelief.
Therefore, as the Holy Spirit says: “Today, if you hear His voice, do not harden your hearts, as you did in the rebellion, in the day of testing in the wilderness, where your fathers tested and tried Me, and for forty years saw My works. Therefore I was angry with that generation, and I said, ‘Their hearts are always going astray, and they have not known My ways.’ So I swore on oath in My anger, ‘They shall never enter My rest.’”
Psalm 95:7-11 recalls the generation that tested God, went astray, and was barred by oath from His rest; Hebrews 3:7-19 quotes the warning and identifies unbelief as the reason they could not enter.