Rashi on Genesis 42:2/רש"י על בראשית מ״ב:ב׳

1רדו שמה GO DOWN THITHER — He did not say to them לכו “Go ye” but רדו an allusion to the 210 years during which Israel was to be enslaved in Egypt corresponding to the numerical value of (רד״ו (210 (Genesis Rabbah 91:2).