David acknowledges in Psalm 51:7 that he is full of sin when he says he is a leper because of his sin. David prays for removal of sin and guilt through hyssop. The allusion is to Leviticus 14:6–7 where the cleansing of a leper is described.1 Hyssop has leaves that are suitable for making a kind of brush, with which you were sprinkled with water. Then you were declared clean.
7 Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow.