It says in Shemos 11:7; “But against all the Children of Yisroel, no dog shall sharpen his tongue, neither against man nor beast, so that you should know that Hashem differentiates between Mitzrayim and between Yisroel.” The Gemara Bava Kama (60B) tells us that when Eliyahu comes to a city the dogs frolic, but when the Angel of Death comes, the dogs whimper.
The Chasam Sofer then asks why were the dogs not whimpering during the Slaying of the Firstborn?
The Chasam Sofer answers that the rule did not apply to the night of Pesach for the Slaying of the Firstborn was administered by Hashem Himself, not through the agency of the Angel of Death as it is written in Shemos 12:12; “And all the Gods of Mitzrayim I will carry out a sentence, I am Hashem.” Rashi explains that what this means is that it was Hashem Himself that carried it out and not through a Shliach.
The fact that the dogs were silent is testimony that it was Hashem who Slayed the Firstborn of Mitzrayim and it was only Hashem that could differentiate between Mitzrayim and Klal Yisroel.
According to the Zera Shimshon the fact that the dogs were silent the night that Hashem Slayed the Firstborns of Mitzrayim gives testimony that Klal Yisroel have been forgiven of their Aveiros and were now indeed worthy of being redeemed.
It says in Shemos Rabbah 1:35 that when Moshe Rabbeinu was a young man and he left the palace for the first time and went out to see Klal Yisroel, Moshe wondered what Klal Yisroel’s Aveira was and why they, out of all the nations of world, had been sentenced to forced labor. Moshe Rabbeinu realized the reason why Klal Yisroel had to suffer such imaginable cruelty is because Dasan and Aviram told Pharaoh that Moshe killed an Egyptian which caused Moshe to flee for his very life. Moshe realized that Klal Yisroel speaks Loshan Hara and therefore, how will Hashem be able to redeem them.
Moshe was in fact very correct to wonder at this point, as the Gemara Makkos (23A) says that anyone who speaks Loshan Hara deserves to be thrown to the dogs.
It says in Devarim Rabbah 2:14 that Klal Yisroel were redeemed from Mitzrayim in the merit of Klal Yisroel doing Teshuvah on speaking Loshan Hara. The proof to this is that the dogs did not chase or bark at Klal Yisroel when they were leaving Mitzrayim. From this it is crystal clear that Klal Yisroel truly did Teshuvah and that they no longer engaged in this terrible Aveirah of Loshan Hara thereby meriting a genuine and honest redemption.
The lesson we must learn out from here is that all of Klal Yisroel is still in Galus because we still speak Loshan Hara about one another. If we all truly want to see and merit the building of the Third and Final Beis HaMikdash, we must cease this behavior. Speaking positively about one another and avoiding speaking Loshan can bring all of Klal Yisroel together as one. Loshan Hara is one thing that counteracts Achdus, but not speaking Loshan Hara and speaking positive things about someone is a true way to do Teshuva for this horrible Aveira and bring a sense of Achdus for all of Klal Yisroel. With this may all Klal Yisroel truly merit the Genuine and honest Final Geulah.