Bilangan 19:6

19:6 And the priest must take cedar wood, hyssop, ¹  and scarlet wool and throw them into the midst of the fire where the heifer is burning. ² 

Bilangan 19:18

19:18 Then a ceremonially clean person must take hyssop, dip it in the water, and sprinkle it on the tent, on all its furnishings, and on the people who were there, or on the one who touched a bone, or one killed, or one who died, or a grave.

[19:6]  ¹ sn In addition to the general references, see R. K. Harrison, “The Biblical Problem of Hyssop,” EvQ 26 (1954): 218-24.

[19:6]  ² sn There is no clear explanation available as to why these items were to be burned with the heifer. N. H. Snaith suggests that in accordance with Babylonian sacrifices they would have enhanced the rites with an aroma (Leviticus and Numbers [NCB], 272). In Lev 14 the wood and the hyssop may have been bound together by the scarlet wool to make a sprinkling device. It may be that the symbolism is what is important here. Cedar wood, for example, is durable; it may have symbolized resistance to future corruption and defilement, an early acquired immunity perhaps (R. K. Harrison, Numbers [WEC], 256).