Halakhah su Numeri 14:1

And again, the entire reason for our present exile is the sin of the spies (viz. Tehillim 6:25-27): "And they murmured in their tents [against the land]. They hearkened not to the word of the L–rd. And He lifted up His hand [in oath] against them… to scatter them in the lands," as Rashi explains there and as the Ramban wrote (on Bamidbar 14:1). And it is stated in Arachin (15a) that the sin of the spies was essentially lashon hara, their giving out an evil report of the land. And because they then (on the eve of the ninth of Av) cried a "vain" cry, it (the ninth of Av) was decreed for them as a "crying" for the generations [e.g., the destruction of both Temples, etc.]. And countless other evils befell us because of this grave sin. For all the sages of Israel who were killed by King Yannai in the days of Shimon ben Shetach, the brother-in-law of King Yannai, were also killed because of the sin of rechiluth (viz. Kiddushin 66a). And the murder of the tanna, R. Elazar Hamodai, (which also contributed to the destruction of Betar) was likewise caused by rechiluth which was spoken against him before Ben Koziva (viz. Eichah Rabbah 2:4).

Furthermore, is it not well known that exile had already been decreed upon us because of the act of the spies (viz. Tehillim 106:25-27): "And they murmured in their tents [against the land]. They hearkened not to the voice of the L–rd. And He lifted up His hand [in oath] against them… to scatter them in the lands," as Rashi and the Ramban explain there (Bamidbar 14). And the sin of the spies — was it not that of lashon hara! (viz. Arachin 15a). Therefore, it is imperative that we correct this sin before the redemption can take place.