As an example of the Yidden's remarkable abstinence from לָשׁוֹן הָרָע, the Midrash notes the following. The commandment to take the wealth of the Egyptians shortly before their exodus from Egypt, was already known to B'nei Yisrael before Moshe went to Pharaoh the first time. The commandment was given to them by Hashem when Moshe was initially sent to redeem them from slavery.
When Hashem first revealed Himself to Moshe at the סְנֶה, the burning thornbush, He instructed him to gather the elders of Yisrael and inform them that their cries had been heard and they would soon be redeemed from Egypt and brought to a land of their own. Their salvation will not be immediate, as the king of Egypt will only let them go when he is forced to do so by experiencing Hashem's strong Hand and His wonders brought upon himself and his nation. However, when the great redemption will finally come, Klal Yisrael will not leave empty-handed. They will take along the riches of Egypt, borrowing from their neighbours and receiving all the Egyptians' wealth.
However, the Midrash says, despite being entrusted with this knowledge twelve whole months earlier, not a single Jew informed the Egyptians. This secret was never revealed to the Mitzrim during the entire period leading up to the actual redemption. This incredible display of unity and restraint significantly contributed to bringing the Geulah closer.
This virtue didn't only play a crucial role in bringing about their redemption, their salvation was actually entirely dependent on it.
When Moshe Rabbeinu reprimanded Dasan and Aviram for hitting one another, they threatened to inform Pharaoh that he had killed an Egyptian. Moshe countered, "אָכֵן נוֹדַע הַדָּבָר", indeed, the matter is known.
The Midrash expounds on this response, explaining that Moshe had been grappling with why the Yidden suffered so terribly under the hands of the Mitzrim without an end in sight. However, upon realizing that there were informers amongst the Jewish people, he understood that this was the sin which caused them to be enslaved. In other words, the sin of לָשׁוֹן הָרָע, which is a manifestation of divisiveness and lack of unity, was the root cause of their ongoing suffering.
(מדרש רבה שמות א ל)