בַּרְזֶ֣ל בְּבַרְזֶ֣ל יָ֑חַד וְ֝אִ֗ישׁ יַ֣חַד פְּנֵֽי־רֵעֵֽהוּ׃
Iron sharpeneth iron; So a man sharpeneth the countenance of his friend.
Rashi on Proverbs
sharpens the countenance of his friend Heb. יחד, sharpens. Torah scholars sharpen each other in halachah.