Yoma 53b
יומא נ״ג ב — ד"ה אָמַר רַבִּי אֲלֶכְּסַנְדְּרִי אָמַר
Yoma 53b:2
One who prays must take three steps backward upon concluding his prayer and then recite Peace; if he does not do so, it is considered as though he did not pray at all, as this action shows proper respect toward God, and he should remain standing rather than immediately return to his original place.
אָמַר רַבִּי אֲלֶכְּסַנְדְּרִי אָמַר רַבִּי יְהוֹשֻׁעַ בֶּן לֵוִי: הַמִּתְפַּלֵּל, צָרִיךְ שֶׁיַּפְסִיעַ שָׁלֹשׁ פְּסִיעוֹת לַאֲחוֹרָיו, וְאַחַר כָּךְ יִתֵּן שָׁלוֹם. אָמַר לֵיהּ רַב מָרְדֳּכַי: כֵּיוָן שֶׁפָּסַע שָׁלֹשׁ פְּסִיעוֹת לַאֲחוֹרָיו — הָתָם אִיבַּעְיָא לֵיהּ לְמֵיקַם. מָשָׁל לְתַלְמִיד הַנִּפְטָר מֵרַבּוֹ, אִם חוֹזֵר לְאַלְתַּר, דּוֹמֶה לְכֶלֶב שֶׁשָּׁב עַל קִיאוֹ. תַּנְיָא נָמֵי הָכִי: הַמִּתְפַּלֵּל, צָרִיךְ שֶׁיַּפְסִיעַ שָׁלֹשׁ פְּסִיעוֹת לַאֲחוֹרָיו וְאַחַר כָּךְ יִתֵּן שָׁלוֹם, וְאִם לֹא עָשָׂה כֵּן רָאוּי לוֹ שֶׁלֹּא הִתְפַּלֵּל. וּמִשּׁוּם שְׁמַעְיָה אָמְרוּ: שֶׁנּוֹתֵן שָׁלוֹם לְיָמִין, וְאַחַר כָּךְ לִשְׂמֹאל, שֶׁנֶּאֱמַר: ״מִימִינוֹ אֵשׁ דָּת לָמוֹ״, וְאוֹמֵר: ״יִפּוֹל מִצִּדְּךָ אֶלֶף וּרְבָבָה מִימִינֶךָ״.
§ Apropos the obligation of a student to walk backward when taking leave of his teacher, the Gemara discusses a similar topic. Rabbi Alexandri said that Rabbi Yehoshua ben Levi said: One who prays must take three steps backward upon concluding his prayer and then recite: Peace, in a manner befitting one who departs from before the Holy One, Blessed be He. Rav Mordekhai said to him: Since he has taken three steps backward, he should stand there and not return to his place immediately. This is analogous to a student who takes leave of his teacher. If he returns immediately to the place where was first standing, he is similar to a dog who returns to its vomit, and his previous action is spoiled. The Gemara comments that this was also taught in a baraita: One who prays must take three steps backward upon concluding his prayer and then recite: Peace. And if he did not do so, it is better for him not to have prayed, as his actions are disrespectful toward God. And they said in the name of Shemaya the Sage that when one recites: Peace, he first bows to the right and then to the left, as it is stated: “At His right hand was a fiery law to them” (Deuteronomy 33:2), and it says: “A thousand may fall at your side, and ten thousand at your right side” (Psalms 91:7).