Hebrews 9:19-20
For why (tell me) is the book of the testament sprinkled? “For” (he says) “when Moses had spoken every precept to all the people according to the law, he took the blood of calves, with water, and scarlet wool, and hyssop, and sprinkled both the book itself and all the people, saying, This is the blood of the testament, which God has enjoined unto you.” Tell me then why is the book of the testament sprinkled, and also the people, except on account of the precious blood, figured from the first? Why “with hyssop”? It is close and retentive. And why the “water”? It shows forth also the cleansing by water. And why the “wool”? This also [was used], that the blood might be retained. In this place blood and water show forth the same thing, for baptism is His passion.
Source: Homilies on the Epistle to the Hebrews (New Advent)