Leviticus 6:4

And he shall take off his vestments and put on other garments, and carry forth the ashes without the camp into a clean place.

AND HE SHALL PUT OFF HIS GARMENTS, [AND PUT ON OTHER GARMENTS, AND CARRY FORTH THE ASHES WITHOUT THE CAMP UNTO A CLEAN PLACE]. the ashes from upon the altar. This was done from time to time when there was a need for it. when so much accumulated that there was no more room on the woodpile, then it was removed to a place outside the camp or Jerusalem. It is this latter subject which is the theme of the present verse. “This was not compulsory for him to do so, but it would be a matter of propriety, so that he should not soil the garments in which he regularly ministers at the altar, by the removal of the ashes to a place without the camp. Garments worn when boiling the pot for one’s master, one should not wear when pouring out a glass of wine for him. That is why He said, and he shall put on other garments — of [a quality] inferior [to those in which he ministers at the altar].” This is Rashi’s language. Now the intent of our Rabbis in this remark was to state that the taking of the ashes [to a place outside the camp] must be done with the priests wearing the priestly garments, and thus the other garments [mentioned in the verse] are not ordinary clothes. Thus I do not know the source for that which the Rabbi [Rashi] said, that “it was not compulsory” [that he remove the garments in which he performs the regular Divine Service, and put on other garments when taking out the ashes to a place outside the camp]. For it would appear that it is a positive commandment to the priest, that the garments in which he performs the acts of offering, including the [daily] lifting up of the ashes, be clean ones, and that he must not perform the regular Divine Service with those garments in which he takes the ashes [outside the camp]. This commandment [of the changing of the garments] is of the nature of a servant’s etiquette towards his master. Therefore the priests should have more expensive garments for performing the acts of offering, and ones of inferior quality for removing the ashes [to a place outside the camp or city of Jerusalem].
All this is in accordance with this reasoning which the Rabbi [Rashi] wrote [i.e., that the removal of the ashes must be done in priestly garments]. But there are some of our Rabbis mentioned there in Tractate Yoma who say that the taking out of the ashes did not have to be done in priestly garments. Thus the verse stating, and he shall put on other garments means “ordinary clothes,” and this is also the plain meaning of the verse, commanding the priest that he should not soil the linen garments, which are the holy garments, with the taking out of the ashes; instead he is to put on ordinary clothes.