Sefer Devarim is Moshe's closing message to Bnei Yisrael where he gives the mussar for the aveiros they have done, reviews with them all the mitzvos, and warns them about the future. The Gemara says (Bava Basra 165a) that most people steal, a few enter illicit relationships, and everybody speaks Avak Lashon Hara.
The Vilna Gaon finds a Remez to this gemara in the opening words of the parsha. "Eileh Hadevarim Asher Diber Moshe El Kol Yisroel". Eileh (Aleph, Lamed, Hey) is for Avak Lashon Hara. That is why it says El Kol Yisrael, because we are all oveir on this.
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