Moussar sur Les Nombres 15:31

And there are many people among the masses who think that the main loss and damage to the soul is only from sins that [involve] an action; and that there is no losing of the soul for a man that is pure from sinning actively and who did not walk in the way of transgressions, but rather [only] prevented himself from doing [positive] commandments and good deeds. Therefore, we are obligated to make discernment known to the ones of a mistaken spirit. For our Rabbis, may their memory be blessed, said (Yerushalmi Chagigah 1:7), “The Holy One, blessed be He, forewent idolatry, etc., but He did not forego the sin of wasting [time available for] Torah [study].” And they said, (Sifre Devarim, Ekev), “Just like the reward for Torah study is greater than [that for] all of the commandments in the Torah, so too is the punishment for wasting [time for] it greater than [that for] all of the sins.” And they said (Sanhedrin 99a), “‘Because he has despised the word of the Lord and has breached His commandment’ (Numbers 15:31) - is stated about anyone for whom it is possible to engage in Torah [study] and does not engage [in it].” And we have already mentioned all of these to you in the Gate of the Torah (no longer extant).

Among the [negative commandments] that are dependent on the (heart) [tongue] are: “Since the Lord, your God, moves about in your camp to protect you and to deliver your enemies to you, let your camp be holy; let not anything unseemly be seen among you” (Deuteronomy 23:15). They, may their memory be blessed, said (Berakhot 25b) that included in this [prohibition] is that when we speak words of the Torah of God, may He be blessed - and in our speaking in prayer in front of Him - our camp should be holy, and let not anything unseemly be seen among us. Therefore we were commanded about this, to mention [God’s] name in holiness and to engage in words of Torah and prayer in holiness; and not to mention [God’s] name or words of Torah if he is naked or if there is a naked man across from him. And likewise were we warned that the place be clean, as it is stated (Deuteronomy 23:14), “you shall dig a hole with it and cover up your excrement.” And all the more so is he warned not to mention [God’s] name if his hands are not clean. And if his hands touched something disgusting to him, he should wash them - like the matter that is stated (Psalms 26:6), “I wash my palms in cleanliness.” And when one is walking on the path and he is in doubt if the path is clean, he should not mention [God’s] name, and he should not say words of Torah. And if there is something that is not clean behind him - such as excrement or a carcass or water [used for] soaking [flax] - one must move a distance of four ells away from where the odor ends; but in front of him, [the distance] is [as far as] his eye can see. And carefulness about this is one of the ways of fearing God, as it is stated (Malachi 3:16), “concerning those who revere the Lord and esteem His name.” And our Rabbis, may their memory be blessed, said (Berakhot 24b), “‘For he has shown contempt for the word of the Lord’ (Numbers 15:31) - this is [referring to] one who says words of Torah in filthy alleyways.” And our Rabbis, may their memory be blessed, said (Kohelet Rabbah 1:17), “‘He gives wisdom to the wise’ (Daniel 2:21) because the wise honor the Torah and occupy themselves with it in holiness.” But if He had given it to the silly, they would have said words of Torah in filthy alleyways.

And our Rabbis, may their memory be blessed, also said (Berakhot 24b) that included in that which is written (Numbers 15:31), “he spurned the word of the Lord,” is one that speaks words of Torah in filthy alleys, anyone who has the possibility of being involved with Torah and is not [so] involved and one who reads the books of heretics; and that, that which it is written, “and he rescinds His commandment” - is [regarding] one who rescinds the covenant of the flesh (circumcision).