Mishnah Parah 3:7: Purification at Sunset

The Mishnah asserts that the Pharisees were able to compel the Sadducees to follow their rulings, even in the Temple, a matter of controversy among modern scholars. In this case, they insisted that the preparation of the ashes of the red heifer (Num. 19) be done by one whom they regarded as fully pure but whom the Sadducees regarded as still being impure until sunset on the final day of his purification.

[The Elders of Israel] used to defile the priest who was to burn the red heifer because of the Sadducees, so that they would not say- [the burning] was done by one upon whom the sun had set.

99. Trans. S. Berrin.