Rashi on Leviticus 19:33/רש"י על ויקרא י״ט:ל״ג

1לא תונו [AND IF A STRANGER SOJOURN WITH THEE IN THY LAND], YE SHALL NOT VEX HIM — This implies vexing him with words (cf. Rashi on Exodus 22:20) — do not say to him, “Yesterday you were an idolator and now you come to study the Torah which was given from the mouth of the Almighty!” (Sifra, Kedoshim, Chapter 8 2; Bava Metzia 58b, 59b.)