בְּזֹ֖את תִּבָּחֵ֑נוּ חֵ֤י פַרְעֹה֙ אִם־תֵּצְא֣וּ מִזֶּ֔ה כִּ֧י אִם־בְּב֛וֹא אֲחִיכֶ֥ם הַקָּטֹ֖ן הֵֽנָּה׃
Dadurch sollt ihr geprüft werden: beim Leben Pharaos, ihr werdet nicht eher von hier weggehen, als bis euer jüngster Bruder hierhergekommen.
Rashi on Genesis
חי פרעה BY THE LIFE OF PHARAOH — If Pharaoh lives! Whenever he swore for the sake of appearance (literally, falsely) he swore by Pharaoh’s life (Genesis Rabbah 91:7).
Sforno on Genesis
בזאת תבחנו, if he is not your brother, the youngest one of whom you spoke will not endanger his own life to come with you and to share your fate of the death penalty.
Or HaChaim on Genesis
בזאת תבחנו, "You will be tested as follows:" Joseph expressed readiness to put their words to the test by the yardstick they themselves had provided, although this was not sufficient proof for their claim that they had come to Egypt to obtain the release of their brother.