Mischna zu Bamidbar 19:3

If the cow refused to go out, they may not take out with it a black one lest people say, "They slaughtered a black cow" nor another red [cow] lest people say, "They slaughtered two." Rabbi Yose says: it was not for this reason but because it is said "And he shall bring her out" by herself. The elders of Israel used to go first by foot to the Mount of Olives, where there was a place of immersion. The priest that was to burn the cow was (deliberately) made unclean on account of the Sadducees so that they should not be able to say, "It can be done only by those on whom the sun has set."