And he [King Hezekiah] set the Levites in the house of the Lord with cymbals, with psalteries, and with harps, according to the commandment of David, and of Gad the king's seer, and Nathan the prophet: for so was the commandment of the Lord by his prophets. (2 Chron 29:25; cf. 2 Chron 35:4)
In my lengthy critique of the Sola Scriptura, the formal doctrine of Protestantism, Not by Scripture Alone: A Latter-day Saint Refutation of Sola Scriptura, I wrote the following about the importance of 2 Chron 29:25:
we learn the following [from 2 Chron 29:25 and its parallel in 35:4]: (1) firstly, David, Gad, and Nathan were dead for about 250 years at this point; however, (2) they passed on a "command . . . from the Lord" which was prescribed by God's prophets on how worship to be conducted in the temple (hardly a minor issue; the worship of God is a central issue in theology) and (3) such a prescription and commandment is nowhere found in the entirety of the Bible.
Sara Japhet, in one of the leading scholarly commentaries on 1-2 Chronicles, offered the following insightful commentary on this verse:
Now the story moves to the other aspect of preparation, that of the music accompanying the sacrifices, but the main interest of the verse lies neither in the enumeration of the participants - which is accomplished by the short 'he stationed the Levites' - nor in the stages of preparation. The verse centres on the problem of authority, presented in two states. The initial command by which music was made an integral part of the sacrificial ritual is attributed to David and the prophets of his time: 'the commandment of David and of Gad the king's seer, and of Nathan the prophet'. Then comes a general statement, the significance of which cannot be overestimated, the addressing in general terms the problem of authority: 'for the commandment is (RSV was) from the Lord through the prophets'. The phrasing of this statement needs some clarification. 'The hand of the Lord' is one of several biblical idioms for divine inspiration (e.g. I Kings 18.46; II Kings 3.15, and in particular Ezekiel - 1.3; 3.22; 8.1, etc.). 'As the Lord has commanded/spoken by the hand of (RSV 'through') a prophet' (Exod. 9.35; 35.29, etc.) is again a recurrent idiom, describing the intermediary role of the prophet in announcing the divine will. The precise phrasing of this verse combines the two idioms 'by the Lord's inspiration' and 'by the hand of the prophet'. (the first beyad, 'by the hand', is probably a corruption by analogy of miyyad, as recognized by practically all commentators, including RSV: 'from the Lord'; cf. I Chron. 28.19.) They are both related to miswāh (commandment or ordinance), a term which eventually became the overarching term for all the Pentateuchal injunctions. Here it occurs in the determinate singular, an abstract noun denoting the phenomenon of 'commandment' as such.
The statement contains several elements which, taken together, constitute an important plank in the Chronicler's theological platform. The plural 'prophets', its coming after the explicit mention of Nathan and Gad, and the causative 'for' (kī), press the point that the Lord's commandment are delivered to the people not exclusively by Moses, but by 'the prophets' in general. Thus, according to the Chronicler, it is not only messages of rebuke or encouragement which are delivered in ad hoc situations by the prophets after Moses; the prophets after Moses also deliver the 'commandment' with its broad sense of legal obligatoriness. The general statement is linked to an institution which is presented clearly and explicitly as an innovation, established for the first time by David. This, then, represents an expression of the view that legislation did not cease with Moses; even within the sacred realm of the cult there was room for change, indicated by the Lord's continuing inspiration through the prophets. One is tempted to speculate here on the implications of this view for the Chronicler's view of 'Law' and 'canon', but these conjectures would carry us far beyond the evidence.
This verse is phrased in common Chronistic style, as a nominal clause, which reinforces its already general quality. By rendering it in the past ('the commandment was'), with 'commandment' denoting an ordinary singular, RSV limits its scope to an actual contemporary situation. Even so, however, it does not eradicate its basic meaning. A general rendering 'is', or 'commandments are', would better represent the original.
Note that Gad is called 'seer' and Nathan 'prophet'; their common titles in Chronicles, and in juxtaposition to the priests, the singers are systematically called 'Levites' (vv. 25, 26, 30) (Sara Japhet, I & II Chronicles [Old Testament Library; Louisville, Ky.: Westminster John Knox Press, 1993],, locations 22954-22992 on the Kindle Version)