The phrase "Word of God" or "Word of the Lord" and variants are commonly used (at least among evangelicals) to refer to the Bible - i.e. the written words of the prophets and apostles. I'm wondering what is the origin of this usage.

These phrases appear frequently in the Bible itself, but never (as far as I'm aware) to unambiguously refer to the written Bible itself. For instance, throughout the prophets it is commonly written "the word of the Lord came to so and so," but this seems to refer to something prior to the actual writing down of the relevant books. In the NT we have for example the oft quoted Heb. 4:12 saying "The Word of God is living and active..." which is often taken as referring to the Scriptures, but this reading doesn't appear necessitated by the context unless we already take the phrase Word of God to mean this. In some cases, it unambiguously refers to something *not* the Bible, e.g. in John 1.

To be clear, I am not asking about where the idea of divine inspiration of the Bible comes from. Rather, I am asking specifically about the origin of using the phrase "word of God" to refer to the Bible.