Kiddushin 70 - Whoever Calls Others Unfit is Himself Unfit

Whoever declares others to be genealogically unfit is himself genealogically unfit, never speaks in praise of others, and declares them unfit with his own blemish.

A man in Pumbedita offended Rav Yehuda. The man used to call other people slaves. Rav Yehuda excommunicated him and pronounced him a slave. In the court of Rav Nachman, Rav Yehudah explained that he pronounced the man slave because he called others slaves. Usually, this only allows one to suspect, but on that day, Rav Matna, who had not visited Pumbedita for 13 years, came and confirmed that the man was a slave, and more blemishes were discovered.

Art: Karoly Ferenczy - Joseph Sold Into Slavery By His Brothers