וַיְהִ֗י כָּל־יֽוֹדְעוֹ֙ מֵאִתְּמ֣וֹל שִׁלְשׁ֔וֹם וַיִּרְא֕וּ וְהִנֵּ֥ה עִם־נְבִאִ֖ים נִבָּ֑א וַיֹּ֨אמֶר הָעָ֜ם אִ֣ישׁ אֶל־רֵעֵ֗הוּ מַה־זֶּה֙ הָיָ֣ה לְבֶן־קִ֔ישׁ הֲגַ֥ם שָׁא֖וּל בַּנְּבִיאִֽים׃
And it came to pass, when all that knew him beforetime saw that, behold, he prophesied with the prophets, then the people said one to another: ‘What is this that is come unto the son of Kish? Is Saul also among the prophets?’
Read midrashic interpretation of I Samuel 10:11: and classical rabbinic exegesis.