Jeremy D. Smoak discusses linguistic evidence for Numbers 6:24-26 (part of the Priestly source) being preexilic in origin.
Jeremy D. Smoak, The Priestly Blessing in Inscription and Scripture: The Early History of Numbers 6:24-26 (Oxford: Oxford University Press, 2016), 86-87
One aspect of the instructions that may point to a preexilic date in the use of the infinitive absolute of the verb ‘mr “say” as a command in verse 23. The function of the infinitive absolute is more common in classical biblical Hebrew than later biblical Hebrew literature. The majority of the uses of the infinitive absolute as a command appear in the literature of Genesis-Exodus and the Deuteornomistic History, and the Latter prophets. As noted earlier in this chapter, the use of the infinitive absolute as a command is particularly evident in the description of divine commands in the Torah. For instance, Exodus 20:8 stipulates the observance of the Sabbath through the verb zkr as an infinitive absolute (zkwr). In a similar manner, Exodus 12:48 employs the infinitive absolute to convey the divine command that the Hebrew circumcise their male children (hmwl). In the prophetic literature, infinitive absolutes appear in several places to convey prophetic commands (2 Sam. 24:12; 2 Kings 3:16; Isa. 14:31).
This contrasts sharply with the general reduction in the use of the infinitive absolute as a command in the literature of the book of Chronicles and other biblical texts from the Second Temple period. The strongest evidence for this argument comes from a comparison of Samuel-Kings and Chronicles with regard to the use of the infinitive absolute form to convey a command. In the places where the author of Samuel-Kings employed an infinitive absolute to convey a command, the authors of Chronicles used the command form of the same verb. Such a move is illustrated by the differences in grammar between 2 Samuel 24:12 and 1 Chronicles 21:10:
Go (hlwk) and say to David . . . (2 Sam 24:12)
Go (ilk) and say to David . . . (1 Chr 21:10)
As the above passages show, the authors of Chronicles chose to render the command in 2 Samuel 24:12 through the use of a command form as opposed to an infinitive absolute form. Such a situation most likely indicates that the use of the infinitive absolute as a command fell out of use in late biblical Hebrew during the postexilic period. This argument is strengthened by the observation that the use of the infinitive absolute as a command in the classical biblical Hebrew texts is matched by a similar usage for the infinitive in preexilic Hebrew texts is matched by a similar usage for the infinitive in preexilic Hebrew inscriptions (Arad 2.1). Such a situation provides some indication that the use of the infinitive absolute as a command was relatively common in the preexilic period, but more limited during the Second Temple Period.