Kommentar zu Schemuel I 13:1

One year was Shaul in his reign. Our Rabbis, of sainted memory, said,¹ Maseches Yoma 22b. [Shaul was] like a one year old, who did not experience the taste of sin. It may also be interpreted, "Shaul was a year in his reign," i.e., in the first year in which he was made king, though he actually ruled over Yisroel two years, in the first year, immediately, "Shaul chose for himself three thousand [men]."