It is difficult to see exactly how this verse is related to its context. The possibilities are as follows:
(a) Good News Translation and other common language translations solve the problem by making it a separate sentence, unrelated to anything else. This is the simplest practical solution and probably the best.
(b) Put verse 14 in round brackets (parentheses) and thus link “while it is said” (Revised Standard Version) with verse 13. This raises almost as many problems as it solves.
(c) Link “while it is said” with hold firmly to the end (verse 14) is a rather awkward way of introducing a repetition of the quotation; compare Moffatt “this word ever sounding in our ears, To-day…” (similarly Traduction œcuménique de la Bible). This is the construction and punctuation chosen by the UBS Greek New Testament, first to third editions.
(d) Some editions of the Greek text, followed by Knox, Jerusalem Bible, and New American Bible, link verses 15 and 16. New American Bible has “When Scripture says, ‘Today, if you should hear his voice, harden not your hearts as at the revolt,’ who were those that revolted when they heard that voice?” This translation rightly assumes that the writer of Hebrews goes on quoting the psalm until he reaches the word “rebel,” because he wishes to comment on that word.

This is what the scripture says must often be restructured, as in other instances, as “This is what one may read in the Scriptures” or “… at one place in the holy writings.”

The content of this passage should be translated in essentially the same manner as these words are rendered in verses 7b-8a.

Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on The Letter of the Hebrews. (UBS Handbook Series). New York: UBS, 1983. For this and other handbooks for translators see here .