Shemirat HaLashon שמירת הלשון

(Ibid. 17:7): "And he called the name of the place Massah and Merivah because of the "quarrel" (riv) of the children of Israel and because of their "proving" (nasotham) the L-rd, saying: 'Is the L-rd in our midst or not?'" This is followed by (8): "And Amalek came and warred, etc." From the juxtaposition of the verses we learn that the sin of Merivah also contributed to bringing Amalek upon them. The same holds true for other idolatries — wherefore one must take great care to avoid merivah (quarreling).

(Ibid. 23:1): "Do not receive a false report," concerning which our sages have said (Pesachim 118a): "All who speak lashon hara deserve to be cast to the dogs, it being written 'Do not receive [tissa] a false report,' which may also be read 'Do not spread [tassi] a false report.' And this is preceded by (22:30): 'To the dog shall you throw it [which can also be read "him"].'" This is easily understood. For it is the way of the dog always to bark and sometimes even to bite one who walks by him, even if he does so "innocently." So is it the way of the man of lashon hara to harm everyone with his "bark." He finds fault with all men and shames them. It is only fit, then, that he be cast before the dogs and that they affright and bite him, "measure for measure."

The Maharal of Prague gives another reason. The dogs guarded themselves when necessary [at the exodus] from barking, as it is written (Ibid. 11;7): "And against all the children of Israel a dog will not sharpen its tongue." Yet he [man] whom the L-rd has accorded understanding and knowledge cannot keep his yetzer from doing so — wherefore he is worse than they!

The Sefer Charedim writes in the name of he Kabbalists that sometimes Heaven reincarnates his soul in a dog, and this is intimated by "He deserves to be cast to the dogs."

Juxtaposed with this is (23:2): "Do not incline after many [wicked men] for evil." The commentators explained this to mean that even if one sees that many have habituated themselves to a certain sin he should not follow them. And so, in our instance. Even if one sees that many of his friends and acquaintances stumble in this sin of lashon hara, he should not be enticed into following them.