בַּרְזֶ֣ל בְּבַרְזֶ֣ל יָ֑חַד וְ֝אִ֗ישׁ יַ֣חַד פְּנֵֽי־רֵעֵֽהוּ׃
Le fer devient poli au contact du fer et l’homme au contact de son prochain.
Taanit
Rabbi Chamma said, (Rabbi) Chanina (said), "What is [the meaning of] what is written (Proverbs 27:17), 'Iron sharpens iron?' To say to you [that] just as [in the case of] iron, one sharpens its fellow; so too two Torah scholars sharpen each other in [matters of] Halacha (law)."