Ezra 6:20

For the priests and the Levites were purified together, all of them were pure, having taken care to perform the rites of consecration, Exodus 19:10, and to be cleansed from all Levitical defilement, and killed the passover for all the children of the captivity, for it gradually became the custom for the Levites to do the slaughtering of all the Passover lambs, since it was so easy for defilement to rest upon the average person in the congregation, and for their brethren, the priests, who were busy at the altar of burnt offering, and for themselves.