According to George Reynolds and Janne Sjodahl, the prophet here in 1 Nephi 13:23 is speaking on the Old Testament, as it was to appear through the labors of Ezra and his associates and successors. Ezra undertook the work of collecting all the sacred writings that existed at his time. This work included not only the discovery of copies in various places, the rejection of those that were not authentic and the copying of manuscripts the contents of which could not otherwise be secured, but also the correction of the text, after careful examination of the variations that must have been found. It was this work that was shown to Nephi in his vision of the Old Testament, and therefore, he, very properly, says he beheld it coming "out of the mouth of a Jew" (1 Nephi 13:23). [George Reynolds and Janne Sjodahl, Commentary on the Book of Mormon, vol. 4, p. 262]
“Behold It Proceedeth Out of the Mouth of a Jew”
According to Cleon Skousen, in 1 Nephi 13:23 the angel seems to be referring primarily to the Old Testament (he gets to the New Testament in verse 24). The statement of the angel that the Old Testament came forth from "a" Jew is highly significant. This undoubtedly refers to the famous Jewish scribe, Ezra, who was the compiler of the Old Testament record which we use in modern times. Ezra was not a prophet but a priest and a scholarly scribe who lived in Babylon. Nearly a century before his time, the Jews had returned to Jerusalem in 538 B.C. and rebuilt the temple of Solomon which was dedicated in 516 B.C. In spite of this, however, Ezra heard that they were not obeying the commandments of God but were desecrating the sacred temple rites with their impure lives. He therefore went to Jerusalem to start a massive reform movement. Once he arrived in Jerusalem, however, Ezra discovered the Jews had lost practically all of their scriptures. It will be recalled that in ancient times the scriptures were maintained as separate books, and one by one these had become lost. Ezra therefore gathered together all of the prophetic writings which he had brought with him as well as those he was able to find in Jerusalem, and compiled them into a single canon of scripture which became our Old Testament. It is estimated that Ezra did most of this work between approximately 450-400 B.C. This allowed him to include the writings of Malachi who lived during the latter part of this period. The Jews then went into 400 years of dark ages so Malachi is the last prophet cited in the Old Testament. In view of these known historical facts it is literally true that the existence of our present Old Testament scripture is the direct result of the careful compilation made by Ezra, and it was entirely accurate for the angel to say that this book proceeded from the mouth of "a" Jew. [W. Cleon Skousen, Treasures from the Book of Mormon, Vol. 1, p. 1106] See the commentary on 1 Nephi 5:11: 5:18]