Tosefta zu Dewarim 21:1

A ma’aseh about two priestly brothers were running next to one another and they were going up the ramp. One of them preceded his fellow to the four amot. One of them took a knife and stabbed him in his heart. Rabbi Tzadoq came stood on the steps of the porch on the Temple Mount and said, “Hear me, my brothers, the house of Israel. Indeed it is said, ‘If someone is found slain in the land’ (Deut 21:1). And us, from where shall we measure? From the Sanctuary or from the courtyard?” All of Israel moaned in tears after his [words]. After that, the father of the boy came. He said to them, “I shall atone for you! My son is [still] convulsing, so the knife has not become impure.” This teaches that the impurity of the knife was more serious to them than the shedding of blood. And so it says, “And also Manasseh spilled innocent blood” (II Kings 21:16). From here , they say, for the sin of bloodshed, the shekhinah was taken away and the Temple was rendered impure.

[What is] valid for the red heifer is invalid for the heifer whose neck is broken (Deut. 21:1-4), and what is valid for the heifer whose neck is broken is invalid for the red heifer (Hul. 1:6). And work [in the field] renders each of them unfit. B'nei yonah (young pigeons), once they are old enough to sip (see Hul. 22b:6), until their feathers become golden [are fit for sacrifice], [while] turtledoves, once they are able to fly, even if they are old [are fit for sacrifice], and consequently it is said, what is valid in turtledoves is invalid in b'nei yonah, and what is valid in b'nei yonah is invalid in turtledoves.