Rom refers to 2 Kings 5: 13-14. Simples!
13 Naaman's servants went to him and said, "My father, if the prophet had told you to do some great thing, would you not have done it? How much more, then, when he tells you, 'Wash and be cleansed'!" 14 So he went down and dipped himself in the Jordan seven times, as the man of God had told him, and his flesh was restored and became clean like that of a young boy.