Shmuel asked Rav Huna: From where do we learn that one who acts upon holy offerings absentmindedly disqualifies them? As it is said, "and he shall slaughter the bull" (Leviticus 1:5), teaching that the slaughter must be for the sake of a bull. He said to him: This is in our hands; from where do we learn that it actually invalidates? He said to him: "You shall sacrifice it for your acceptance" (Leviticus 19:5), meaning sacrifice it with full awareness. Rav Huna said: From where do we learn that slaughter is performed at the neck? As it is said, "and he shall slaughter the bull" (Leviticus 1:5), from the place where it bows down, cleanse it. The school of Rabbi Yishmael taught: Do not read "and he shall slaughter" but "and he shall squeeze" [reading the consonants as v'sachat], meaning from the place where it bends, cleanse it. And what is the implication that the word "cleanse it" carries a sense of purification? As it is written, "and he shall cleanse the house" (Leviticus 14:52). And if you wish to say it from here: "Purge me with hyssop, and I shall be clean" (Psalms 51:9).
The Knife at the Neck and the Mind That Must Be Present
Yalkut Shimoni on Torah 440:1
בָּעָא מִנֵּיהּ שְׁמוּאֵל מֵרַב הוּנָא מִנַּיִן לְמִתְעַסֵּק בְּקָדָשִׁים שֶׁהוּא פָּסוּל, שֶׁנֶּאֱמַר "וְשָׁחַט אֶת בֶּן הַבָּקָר", עַד שֶׁתְּהֵא שְׁחִיטָה לְשֵׁם בֶּן בָּקָר. אָמַר לֵיהּ זוֹ בְּיָדֵינוּ הוּא לְעַכֵּב מִנַּיִן אָמַר לֵיהּ (ויקרא יט, ה) "לִרְצֹנְכֶם תִּזְבָּחֻהוּ", לְדַעְתְּכֶם זִבְחוּ. אָמַר רַב הוּנָא מִנַּיִן לִשְׁחִיטָה שֶׁהִיא מִן הַצַּוָּאר שֶׁנֶּאֱמַר "וְשָׁחַט אֶת בֶּן הַבָּקָר", מִמָּקוֹם שֶׁשָּׁח חַטְּאֵהוּ דְּבֵי רַבִּי יִשְׁמָעֵאל תָּנָא אַל תִּקְרֵי 'וְשָׁחַט', אֶלָּא 'וְסָחַט', מִמָּקוֹם שֶׁסָּח חַטֵּהוּ וּמַאי מַשְׁמַע דְּחַטֵּהוּ לִישְׁנָא דִּדְכוּיֵי הוּא דִּכְתִיב "וְחִטֵּא אֶת הַבַּיִת" וְאִי תֵימָא מֵהָכָא (תהלים נא, ט) "תְּחַטְּאֵנִי בְאֵזוֹב וְאֶטְהָר".