The classic source that pikuach nefesh overrides Shabbat, demonstrating the overriding force of saving life in halakhah. It is highly relevant as a conceptual base for life-saving organ donation.
אֲבָל לְהַחֲיוֹת — אֲפִילּוּ מֵעַל מִזְבְּחִי. וּמָה זֶה, שֶׁסָּפֵק יֵשׁ מַמָּשׁ בִּדְבָרָיו סָפֵק אֵין מַמָּשׁ בִּדְבָרָיו, וַעֲבוֹדָה דּוֹחָה שַׁבָּת — קַל וָחוֹמֶר לְפִקּוּחַ נֶפֶשׁ שֶׁדּוֹחֶה אֶת הַשַּׁבָּת. נַעֲנָה רַבִּי אֶלְעָזָר וְאָמַר: וּמָה מִילָה שֶׁהִיא אֶחָד מִמָּאתַיִם וְאַרְבָּעִים וּשְׁמוֹנָה אֵיבָרִים שֶׁבָּאָדָם דּוֹחָה אֶת הַשַּׁבָּת — קַל וָחוֹמֶר לְכׇל גּוּפוֹ שֶׁדּוֹחֶה אֶת הַשַּׁבָּת. רַבִּי שִׁמְעוֹן בֶּן מְנַסְיָא אוֹמֵר: ״וְשָׁמְרוּ בְנֵי יִשְׂרָאֵל אֶת הַשַּׁבָּת״, אָמְרָה תּוֹרָה: חַלֵּל עָלָיו שַׁבָּת אַחַת כְּדֵי שֶׁיִּשְׁמוֹר שַׁבָּתוֹת הַרְבֵּה. אָמַר רַב יְהוּדָה אָמַר שְׁמוּאֵל: אִי הֲוַאי הָתָם, הֲוָה אָמֵינָא דִּידִי עֲדִיפָא מִדִּידְהוּ: ״וְחַי בָּהֶם״ — וְלֹא שֶׁיָּמוּת בָּהֶם.
but to preserve a life, e.g., if the priest can testify to the innocence of one who is sentenced to death, one removes him even from on top of My altar, even while he is sacrificing an offering. Just as this priest, about whom there is uncertainty whether there is substance to his words of testimony or whether there is no substance to his words, is taken from the Temple service in order to save a life, and Temple service overrides Shabbat, so too, a fortiori, saving a life overrides Shabbat. Rabbi Elazar ben Azarya answered and said: Just as the mitzva of circumcision, which rectifies only one of the 248 limbs of the body, overrides Shabbat, so too, a fortiori, saving one’s whole body, which is entirely involved in mitzvot, overrides Shabbat. Rabbi Shimon ben Menasya said: It is stated: “And the children of Israel shall keep Shabbat, to observe Shabbat” (Exodus 31:16). The Torah said: Desecrate one Shabbat on his behalf so he will observe many Shabbatot. Rav Yehuda said that Shmuel said: If I would have been there among those Sages who debated this question, I would have said that my proof is preferable to theirs, as it states: “You shall keep My statutes and My ordinances, which a person shall do and live by them” (Leviticus 18:5), and not that he should die by them. In all circumstances, one must take care not to die as a result of fulfilling the mitzvot.