Yoma, Chapter 3, Mishnah 3. In the previous mishnah we learned several rules concerning immersion in the mikvah in the Beis HaMikdash, and now the mishnah teaches us an additional rule.
The rule: one does not enter the Azarah without immersing:
A person may not enter the Azarah - the courtyard of the Beis HaMikdash - to perform the service, unless he has first immersed. The commentators explain that although the mishnah speaks of entering in order to perform the service, the rule applies even to someone who is not performing any service, and even to a Yisrael who enters. And even a person who knows about himself that he is tahor may not enter until he immerses in a mikvah before entering the courtyard of the Beis HaMikdash.
The immersions of the Kohen Gadol on Yom Kippur:
The mishnah teaches a rule concerning the immersions of the Kohen Gadol on Yom Kippur: "Chamesh tevilos va'asarah kidushin toveil Kohen Gadol u'mekadeish bo bayom" - on that very day the Kohen Gadol was required to perform two sets of acts:
Five immersions - five times he immersed in a mikvah.
Ten sanctifications - ten times he sanctified his hands and feet, by pouring water over his hands and feet.
"V'chulan bakodesh al beis haParvah" - all of the immersions were performed in a holy place, in an area sanctified with the sanctity of the Azarah. Their location was in a place called the Parvah Chamber - the name of a room, and on its roof was the immersion area in which the Kohen Gadol immersed.
"Chutz mizu bilvad" - except for the first immersion mentioned in the previous mishnah, when the Kohen Gadol was led to immerse before the service of the daily Tamid offering. That immersion did not need to be performed in a holy place, and the reason is simple: it is based on the rule that anyone entering the Azarah immerses before entering, and therefore by its very definition it preceded the entry into the Azarah and was not performed in a holy place.
In summary: In this mishnah we learned the rule that one does not enter the Azarah - not even a person who is tahor, and even when not for the purpose of the service - without immersing beforehand; and the count of the immersions of the Kohen Gadol on Yom Kippur: five immersions and ten sanctifications of the hands and feet, all of which were performed in a holy place on the roof of the Parvah Chamber, except for the first immersion, which preceded the entry into the Azarah and therefore was not performed in a holy place.