Musar על שבת 61:9

And you already know the account of Hillel and Shammai (Shabbath 31a) that the three converts to Judaism said concerning them, "The short-temperedness of Shammai wanted to drive us out from the world, however, the modesty of Hillel brought us under the wing of the Divine Presence" As for Hillel, because of his great modesty, no man could make him angry, for he who holds himself back from anger, will acquire the qualities of modesty and compassion, while from wrath comes the quality of cruelty, as it is written : "And my anger shall blaze forth and I will put you to the sword" (Ex. 22:23). And always in the case of anger we find vengeance, "For the Lord's anger will flare up against you and He will shut up the skies" (Deut. 11:17).