וְלָקַ֣ח הַכֹּהֵ֗ן עֵ֥ץ אֶ֛רֶז וְאֵז֖וֹב וּשְׁנִ֣י תוֹלָ֑עַת וְהִשְׁלִ֕יךְ אֶל־תּ֖וֹךְ שְׂרֵפַ֥ת הַפָּרָֽה׃
And the priest shall take cedar-wood, and hyssop, and scarlet, and cast it into the midst of the burning of the heifer.
Read kabbalistic (mystical) interpretation of Numbers 19:6: and classical mystical sources.