Yoma, Chapter 1, Mishnah 5. Here we continue with the High Priest's preparations on the eve of Yom Kippur. The Mishnah opens: "Mesarhu ziknei beis din l'ziknei kehunah, v'he'eluhu la'aliyas beis Avtinas" - the elders of the court handed him over to the elders of the priesthood, and they brought him up to the upper chamber of the House of Avtinas. The elders of the court - those same elders who had been with him throughout the seven days of separation and had reviewed with him the passages of the Torah relating to Yom Kippur - handed him over to the elders of the priesthood, who brought him up to the chamber in the place called the House of Avtinas. The members of the family of Avtinas, as we learned in Tractate Shevuos, were the ones appointed over the preparation of the incense: the incense used every day, and especially the incense that would be used on Yom Kippur.
The content of the oath:
The Mishnah continues: "V'hishbiuhu" - and they administered an oath to him, without specifying what the oath was about. The subject of the oath lies in a dispute about the order of the incense service: according to the understanding of the Oral Torah, the High Priest is to place the incense on the coals in the fire pan inside the Holy of Holies, and not outside it. The Sadducees, who did not believe in the Oral Torah, held that the incense should be placed on the fire pan before entering, so that he would enter with it already burning. In the era of the Second Temple, when the High Priests were not necessarily righteous men, it was necessary to make certain that the priest was not a Sadducee - and therefore they made him swear that he would perform the service properly, that is, only after entering the Holy of Holies.
After the oath: "V'niftru v'halchu lahem" - the elders of the court took leave of him and went on their way, and did not remain with the elders of the priesthood.
The wording of the oath as the Mishnah describes it:
"Ishi kohen gadol" - our man, the High Priest.
"Anu shluchei beis din, v'atah shluchenu ushliach beis din" - we are the agents of the Sanhedrin, and you too are our agent and the agent of the court.
"Mashbi'in anu alecha b'mi sheshiken shemo babayis hazeh, shelo teshaneh davar mikol mah she'amarnu lach" - the oath is taken in the name of the Holy One, Blessed is He, that he not change anything from all that they instructed him, but rather perform the service exactly as the halachah prescribes.
"Hu peirash uvachah, v'hem peirshu uvachu":
Once they finished administering the oath to him, he turned aside and wept, and they too turned aside and wept. Why did both sides weep?
The High Priest: he wept because they suspected him of being a Sadducee, and that is a dreadful suspicion.
The elders of the priesthood: they wept because it was possible that they were 'suspecting the innocent' and had cast suspicion on a person who had done no wrong. And our Sages said: "One who suspects the innocent is stricken in his body" - one who suspects his fellow unjustly is stricken in his body and suffers himself. Therefore, if the priest truly was not a Sadducee, they were distressed over what they had done, and they wept.
In summary: in this Mishnah we learned about the handing over of the High Priest by the elders of the court to the elders of the priesthood and his being brought up to the upper chamber of the House of Avtinas, those appointed over the incense; about the oath they administered to him - that he not deviate from the order of the service established by the Oral Torah, as opposed to the view of the Sadducees who held that the incense should be burned before entering the Holy of Holies; and about the weeping of both sides - he over the suspicion cast upon him, and they over the possibility that they had suspected the innocent.