Reference for Yoma 1:1
שִׁבְעַת יָמִים קֹדֶם יוֹם הַכִּפּוּרִים מַפְרִישִׁין כֹּהֵן גָּדוֹל מִבֵּיתוֹ לְלִשְׁכַּת פַּלְהֶדְרִין, וּמַתְקִינִין לוֹ כֹהֵן אַחֵר תַּחְתָּיו, שֶׁמָּא יֶאֱרַע בּוֹ פְסוּל. רַבִּי יְהוּדָה אוֹמֵר, אַף אִשָּׁה אַחֶרֶת מַתְקִינִין לוֹ, שֶׁמָּא תָמוּת אִשְׁתּוֹ, שֶׁנֶּאֱמַר (ויקרא טז) וְכִפֶּר בַּעֲדוֹ וּבְעַד בֵּיתוֹ. בֵּיתוֹ, זוֹ אִשְׁתּוֹ. אָמְרוּ לוֹ, אִם כֵּן, אֵין לַדָּבָר סוֹף:
Seven days before Yom Kippur the high-priest is separated from his house to the cell of falhedrin. [For all the services of Yom Kippur are kasher only with him, it being written in respect to Yom Kippur (Leviticus 16:32): "And the priest shall make atonement, who shall be anointed." And this separation is derived from what is written in respect to the seven days of consecration, viz. (Ibid. 8:33): "And from the door of the tent of meeting you shall not go out seven days," followed by (34): "As He did on this day, the L rd has commanded to do, to make atonement for you," which our rabbis expounded: "to do" — this alludes to the (procedure with the) red heifer; "to make atonement for you" — this alludes to the Yom Kippur service, viz.: The priest who burns the red heifer and the priest who performs the Yom Kippur service must both be separated from their houses for seven days, as Aaron and his sons required separation on the seven days of consecration. ("to the cell of falhedrin":) The appointees of the king are called "falhedrin." And because the high-priests in the second Temple after Shimon Hatzaddik would pay to serve in the high-priesthood, and, because they were wicked, would not last out their year and would be replaced every twelve months as the king's appointees were — for that reason this cell was called "the cell of the falhedrin."] And another priest is designated "stand-by" [to be high-priest] in the event of his becoming unfit [through keri (an emission) or through uncleanliness (tumah).] R. Yehudah says: A "stand-by wife" is also designated for him in the event that his wife dies, it being written (Leviticus 16:6): "And he shall make atonement for himself and for his house" — his "house" is his wife. Whereupon they said to him: If so, there is no end to the matter! [i.e., If you are apprehensive as to her dying, there is no end to the matter. For perhaps this one, too, (the "stand-by") will die! Rather, with tumah, which is common, we are apprehensive, for which reason we designate another priest as a stand-by. With sudden death, which is not common, we are not apprehensive, for which reason another woman is not designated as a stand-by. The halachah is in accordance with the sages.]