Albert Barnes Commentary Hebrews 3:15

"while it is said, To-day if ye shall hear his voice, Harden not your hearts, as in the provocation." — Hebrews 3:15 (ASV)

While it is said, To-day, and so forth. This means to persevere as long as life lasts, or as long as it can be said "To-day." By persevering in this manner, you will have evidence that you are the friends of the Redeemer. This is a quotation from Psalm 95:7. Paul undoubtedly means to use this language himself as a direct exhortation to the Christians to whom he was writing. He therefore entreats them, as long as it could be said "To-day," or as long as life lasted, to take care that they do not harden their hearts, as had been done in the temptation in the wilderness.