"And you shall not oppress a stranger" (Exodus 23:9). Rabbi Eliezer said: Why did the Torah warn concerning the stranger in some thirty-six places? Because his inclination is bad — lest he return to his former ways [his prior corruption]. "For you were strangers" — from here Rabbi Eliezer said: the blemish that is in you, do not speak of in your fellow. (Note: the manuscript names the first speaker as Rabbi Levi or Rabbi Eliezer; reading uncertain.)
Do Not Oppress the Stranger for You Were Strangers
Mekhilta DeRabbi Shimon Ben Yochai 23:9
וגר לא תלחץ אמר ר' (לוי) [אליעזר] מפני מה הזהירה תורה על הגר כשלשים וששה מקומות מפני שסורו רע שלא יחזור לסורו: כי גרים הייתם. מכאן אמר ר' אליעזר מום שבך אל תאמרו בחבירך: