Pesachim 7b
פסחים ז׳ ב — ד"ה דְּכוּלֵּי עָלְמָא מִיהָא
Pesachim 7b:9
The Talmud articulates the general principle of reciting a blessing עובר לעשייתן, before performing the mitzvah, while discussing immersion and related mitzvah blessings. This is the central Talmudic basis for timing the blessing.
דְּכוּלֵּי עָלְמָא מִיהָא, מֵעִיקָּרָא בָּעִינַן לְבָרוֹכֵי, מְנָלַן? דְּאָמַר רַב יְהוּדָה אָמַר שְׁמוּאֵל: כׇּל הַמִּצְוֹת מְבָרֵךְ עֲלֵיהֶן עוֹבֵר לַעֲשִׂיָּיתָן. בֵּי רַב אָמְרִי: חוּץ מִן הַטְּבִילָה וְשׁוֹפָר. בִּשְׁלָמָא טְבִילָה — דְּאַכַּתִּי גַּבְרָא לָא חֲזֵי. אֶלָּא שׁוֹפָר מַאי טַעְמָא? וְכִי תֵּימָא מִשּׁוּם דִּילְמָא מִיקַּלְקְלָא תְּקִיעָה. אִי הָכִי, אֲפִילּוּ שְׁחִיטָה וּמִילָה נָמֵי! אֶלָּא אָמַר רַב חִסְדָּא: חוּץ מִן הַטְּבִילָה בִּלְבַד אִיתְּמַר. תַּנְיָא נָמֵי הָכִי: טָבַל וְעָלָה, בַּעֲלִיָּיתוֹ אוֹמֵר: ״בָּרוּךְ ... אֲשֶׁר קִדְּשָׁנוּ בְּמִצְוֹתָיו וְצִוָּנוּ עַל הַטְּבִילָה״.
The Gemara poses a question: In any event, it is clear from the previous discussion that everyone agrees that one is required to recite a blessing prior to performing a mitzva. From where do we derive this principle? It is as Rav Yehuda said that Shmuel said: With regard to all the mitzvot, one recites a blessing over them prior to [over] their performance. In the school of Rav they say: One recites a blessing prior to performing all mitzvot, except for the ritual immersion after a nocturnal emission and the blowing of the shofar. The Gemara elaborates: Granted one does not recite a blessing prior to immersion, as this man who has not yet immersed is still unfit to recite a blessing because he is ritually impure. However, with regard to a shofar, what is the reason that one does not recite a blessing before sounding the shofar? And lest you say the reason is due to a concern lest the sounding of the shofar emerge flawed, and the blessing will be in vain, if so, one should not recite a blessing even prior to ritual slaughter and circumcision, as in those cases too one might fail to perform the action in the requisite manner. Rather, Rav Ḥisda said: Except for prior to immersion alone was stated, due to the aforementioned reason. The Gemara adds: That was also taught in a baraita: With regard to one who immersed for ritual purification after a nocturnal emission and emerged, as he emerges he recites: Blessed…Who has made us holy through His mitzvot and commanded us concerning immersion.
