כִּֽי־יִהְיֶ֤ה בְךָ֙ אִ֔ישׁ אֲשֶׁ֛ר לֹא־יִהְיֶ֥ה טָה֖וֹר מִקְּרֵה־לָ֑יְלָה וְיָצָא֙ אֶל־מִח֣וּץ לַֽמַּחֲנֶ֔ה לֹ֥א יָבֹ֖א אֶל־תּ֥וֹךְ הַֽמַּחֲנֶֽה׃
너희 중에 누가 밤에 몽설함으로 부정하거든 진 밖으로 나가고 진 안에 들어오지 아니하다가
Shaarei Teshuvah
“Be on your guard against anything bad” (Deuteronomy 23:10). They, may their memory be blessed, explained (Ketuvot 46a), “A person should not think [impure] thoughts by day and thereby come to impurity by night” - and even though his thought is not to [follow it up with action]. And it is something learned from its [context]. As after it, it is written (Deuteronomy 23:11), “If anyone among you has been rendered unclean by a nocturnal emission.”