Midrash Rabbah Eichah on Scholars of Torah

"Rabbi Yehudah haNasi [the redactor of the Mishnah] would send Rabbi Assi and Rabbi Ami to go forth and rectify the cities of the Land of Israel. They would enter the cities, and they would say to the townsfolk, 'Bring the protectors of the city to us.' The townsfolk would bring the head of the town guard and the watchman to them. They would say to the townsfolk, 'These are the protectors of the city? These are the destroyers of the city!' The townsfolk would then say to Rabbi Assi and Rabbi Ami, 'And who then are the protectors of the city?' The two scholars would say to the townsfolk, 'The guards of the city are the teachers of Scripture and the teachers of Mishnah, for they are studying and learning and upholding the Torah, by day and by night, in accordance with that which Scripture states, 'This book of the Torah shall not depart from your mouth; rather, you should study it day and night' (Joshua 1:8). And so the verse states, 'If YHVH will not build the house, in vain do its builders labor on it; if YHVH will not guard the city, in vain is the watchman vigilant' (Psalms 127:1)."

-- Midrash Rabbah, Eichah 1.2