Leviticus 16:19. As part of God's instructions for Aaron to make atonement for himself, his house, and the people, Aaron is told "sprinkle some of the blood upon [the altar] with his finger seven times and cleanse it and hallow it from the uncleanness of the people of Israel."

This part of the ritual Aaron must perform includes the sprinkling of the blood of a goat sacrificed as a sin offering, but the ritual also includes the sprinkling of the blood of a sacrificed bull. Blood was incredibly important in Jewish ritual, it contains the life of the animal (thus, they were not allowed to consume any meat with blood left in it). By sprinkling the blood on things to make them holy, Aaron will be offering life to God and atonement for His people's sins.

This is a foreshadowing of Jesus offering His blood, pouring it out in His passion and death, on and for all of us. The Jews in the crowd before Pilate even say "His blood be on us and our children." Matthew 27:25, not realizing they are asking for the very thing that Christ wants to give them - the sprinkling of His life life's blood on His people to cleanse them and make them holy.