מדרש על מלכים א 18:45

Variantly: "My taking shall drip as the rain": Just as the rain is not seen until it comes, viz. (I Kings 18:45) "It happened in the meantime that the heavens had darkened with clouds and (filled) with wind, and there was a great rain" — so Torah scholars are not known until they teach Mishnah, halachoth, and aggadoth, or until they are appointed community leaders.