Mishnayos Parah Perek 8 Mishnah 2

One who prepares the mixture of the hatat should not wear his sandals, for were some of the liquid to fall on his sandal it would become unclean and [the sandal] would defile him. Behold he would say [to the sandal], "That which defiled you did not defile me, but you did defile me." If some of the liquid fell on his skin he remains clean. If it fell on his garment it becomes unclean and defiles him. Behold he would say [to the garment], "That which defiled you did not defile me, but you did defile me."