Midrash Tanchuma, Bereshit 8

‎וַיֹּאמֶר ה' אֱלֹהִים. יְלַמְּדֵנוּ רַבֵּנוּ, הַמְסַפֵּר לָשׁוֹן הָרַע מֶה עָנְשׁוֹ? כָּךְ שָׁנוּ רַבּוֹתֵינוּ, הַמְסַפֵּר לָשׁוֹן הָרַע, חָמוּר מִן הָעוֹשֶׂה מַעֲשֶׂה, שֶׁלֹּא נֶחְתַּם גְּזַר דִּין עַל אֲבוֹתֵינוּ בַּמִּדְבָּר אֶלָּא עַל לָשׁוֹן הָרַע, שֶׁנֶּאֱמַר: וַיְנַסּוּ אֹתִי זֶה עֶשֶׂר פְּעָמִים (במדבר יד, כב). And the Lord said: Behold, man has become like one of us (Gen. 3:22). May our master teach us the punishment inflicted upon one who speaks evil?²⁹Speaking evil (slander) is considered the most serious of all sins because it causes the blood to flow to the cheek of the victim. Cf. Arukh 15a. Thus do our masters teach us: One who speaks evil is punished more severely than one who does an evil act. The punishment inflicted upon our ancestors befell them only because of speaking evil, as it is said: You have put me to proof these ten times, and have not hearkened to My voice (Num. 14:22).

אָמַר רַבִּי מָנָא דִּשְׁאָב בְּשֵׁם רַבִּי יְהוֹשֻׁעַ בֶּן לֵוִי, אֵין אָדָם אוֹמֵר לָשׁוֹן הָרַע עַד שֶׁכּוֹפֵר בָּעִקָּר, שֶׁנֶּאֱמַר: אֲשֶׁר אָמְרוּ לִלְשֹׁנֵנוּ נַגְבִּיר שְׂפָתֵינוּ אִתָּנוּ מִי אָדוֹן לָנוּ (תהלים יב, ה). R. Mana of She’ar said in the name of R. Joshua the son of Levi: No man speaks evil until he denies the basic principle of faith (the Oneness of God), as it is said: Our tongue will we make mighty, our lips are with us: who is the Lord over us? (Ps. 12:5).

רַבָּנָן אַמְרֵי, קָשֶׁה לָשׁוֹן הָרַע, שֶׁהֵבִיא מִיתָה עַל אָדָם הָרִאשׁוֹן, שֶׁעָמַד נָחָשׁ וְאָמַר לְאָדָם וְחַוָּה, כִּי יֹדֵעַ אֱלֹהִים כִּי בְּיוֹם אֲכָלְכֶם מִמֶּנּוּ וְנִפְקְחוּ עֵינֵיכֶם (בראשית ג, ה), שֶׁמִּן הָאִילָן הַזֶּה אָכַל כְּשֶׁבָּרָא אֶת עוֹלָמוֹ, וְכָל אֻמָּן שׂוֹנֵא אֶת בְּנֵי אֻמָּנוּתוֹ, וִהְיִיתֶם כֵּאלֹהִים (בראשית ג, ה). שָׁמְעוּ לוֹ, וְגָרְמוּ מִיתָה לָהֶם וּלְתוֹלְדוֹתֵיהֶם עַד סוֹף כָּל הַדּוֹרוֹת. מִנַּיִן, מִן מַה שֶּׁקָּרְאוּ בְּעִנְיַן הֵן הָאָדָם, וְאֵין הֵן אֶלָּא מִיתָה, שֶׁנֶּאֱמַר: הֵן קָרְבוּ יָמֶיךָ לָמוּת (דברים לא, יד). The rabbis said: Slander is considered so grievous a sin that death was imposed upon Adam because of it. The serpent appeared and told Adam and Eve: “God doth know that in the day ye eat thereof, then your eyes shall be open,and ye shall be as God (Gen. 3:5). It was the fruit of this very tree that He ate when He created this world. Now, every craftsman hates a rival craftsman, but if you should eat of this tree, you too shall become divine.”³⁰Cf. Midrash on Psalms 1:9, Bereshit Rabbah 19:4. They listened to him and thus brought death upon themselves and their descendants unto the end of all generations. Whence do we know this? We know it from the words Behold man (Gen. 3:22). The word Behold alludes only to death, as it is written: Behold, thy days approach that thou must die (Deut. 31:14).