Now [Moses] declares that the ashes of the victims (which ought to be taken as a great mystery) are "the sprinkled ashes of a red heifer," which (as the apostle also bears witness) sanctified "those who have been defiled, so that [their] flesh is made clean." He also understands that the sacrament of the Lord's passion, which saves us by purifying us forever, is prefigured in these ashes. Thus the burning of a red heifer designates the actual time and event of Christ's passion, and the burnt ashes which were kept for the cleansing of those who were unclean suggest the mystery of that same passion which has already been completed, by which we are daily purged from our sins.
Source: On the Tabernacle 2.11