(Exodus 30:18-21) It is written, 'Go to the people and sanctify them' (Exodus 19:10). Our masters taught: 'and sanctify them' refers to immersion. We learn the washing of hands from Moses, Aaron, and his sons, as it is said, 'And you shall make a basin of bronze... and they shall wash their hands and their feet.' And concerning Israel, what does it say? 'And you shall sanctify yourselves and you shall be holy.' From here Rabban Gamliel used to eat ordinary food in a state of ritual purity. He said: holiness was given at Sinai not to the priests alone, but to all Israel, as it is said, 'Speak to the whole congregation of the children of Israel and say to them, You shall be holy' (Leviticus 19:2). From here they said: anyone who eats without washing his hands, it is a bad sign for him, and of him it says, 'and it shall be, when he hears the words of this curse... the LORD will not be willing to forgive him.' From here they said: anyone who eats without the washing of hands is as one who has relations with a married woman.
Washing the Hands and the Holiness of All Israel
Yalkut Shimoni on Torah 386:15
(שמות ל יח-כא) כְּתִיב (שמות יט, י) "לֵךְ אֶל הָעָם וְקִדַּשְׁתָּם". שָׁנוּ רַבּוֹתֵינוּ. "וְקִדַּשְׁתָּם", לִטְבִילָה. לָמַדְנוּ רְחִיצַת יָדַיִם מִמֹּשֶׁה וְאַהֲרֹן וּבָנָיו, שֶׁנֶּאֱמַר וְעָשִׂיתָ כִּיּוֹר נְחֹשֶׁת וְגוֹ' וְרָחֲצוּ יְדֵיהֶם וְרַגְלֵיהֶם. וּבְיִשְׂרָאֵל מַהוּ אוֹמֵר "וְהִתְקַדִּשְׁתֶּם וִהְיִיתֶם קְדֹשִׁים". מִכָּאן הָיָה רַבָּן גַּמְלִיאֵל אוֹכֵל חֻלִּין בְּטָהֳרָה, אָמַר, לֹא לְכֹהֲנִים בִּלְבַד נִתְּנָה קְדֻשָּׁה מִסִּינַי אֶלָּא לְכָל יִשְׂרָאֵל, שֶׁנֶּאֱמַר (שם יט, ב) "דַּבֵּר אֶל כָּל עֲדַת בְּנֵי יִשְׂרָאֵל וְאָמַרְתָּ אֲלֵיהֶם קְדֹשִׁים תִּהְיוּ". מִכָּאן אָמְרוּ, כָּל הָאוֹכֵל בְּלֹא רְחִיצַת יָדַיִם סִימָן רַע לוֹ, וְעָלָיו הוּא אוֹמֵר "וְהָיָה [בְּ](כְּ)שָׁמְעוֹ אֶת דִּבְרֵי הָאָלָה וְגוֹ' לֹא יֹאבֶה ה' סְלֹחַ לוֹ". מִכָּאן אָמְרוּ, כָּל הָאוֹכֵל בְּלֹא נְטִילַת יָדַיִם כְּבָא עַל אֵשֶׁת אִישׁ.