Parashas Acharei Mos · Passuk 16:04
Chabad.org — English translation + Rashi
Torah Temima points to Yoma 32a.
English: R’ said: From where [do we know] five immersions and ten sanctifications that the Kohen Gadol immerses and sanctifies on that day? From “a sacred linen tunic shall he wear… and he shall wash… and put them on,” and “they are sacred garments” — all the garments are equated to one another.
Five tevilot and ten kiddushin
Each change of garments on Yom Kippur requires immersion and sanctification; the five changes yield five immersions and ten hand-washings.