Charles Ellicott Commentary Psalms 51:7

"Purify me with hyssop, and I shall be clean: Wash me, and I shall be whiter than snow." — Psalms 51:7 (ASV)

Hyssop. —The mention of this connects this verse with the priestly ordinances concerning leprosy and contact with a dead body (Leviticus 14; Numbers 19); but generally it is a repetition of the former prayer to have the breach made in the covenant relationship healed .