“Saul himself was a Benjaminite—Rabbinic sources identified him with the anonymous refugee in the present story—and the little tribe plays a big role in the stories of this period” (P. Kyle McCarter Jr., I Samuel: A New Translation with Introduction, Notes and Commentary, AB 8 [New Haven: Yale University Press, 2008], 113).Tsumura likewise references McCarter.
“Benjamin’s territory was between Jerusalem and Bethel. Saul was a Benjamite, and rabbinic traditions identified this messenger as Saul” (David Tsumura, The First Book of Samuel, NICOT [Grand Rapids: Eerdmans, 2007], 197).Klein adds some additional detail.
“The Rabbis thought that the Benjaminite messenger of v 12 was Saul, who had rescued the tablets of the law from Goliath, but the story teller himself leaves us uninformed about the significance of the messenger’s tribal identity” (Ralph W. Klein, 1 Samuel, WBC 10 [Dallas: Word, 1998], 43).
But note that none of these references have identified a/the source for this rabbinic tradition. Because of other projects I don’t have time to research it further. But if you do and find it, please leave me a comment.