John Gill Commentary 2 Kings 5:13

"And his servants came near, and spake unto him, and said, My father, if the prophet had bid thee do some great thing, wouldest thou not have done it? how much rather then, when he saith to thee, Wash, and be clean?" — 2 Kings 5:13 (ASV)

And his servant came near, and spoke to him, and said, my
father
Or my lord, as the Targum; this being not a familiar and affectionate expression, but a term of honour, reverence, and submission:

if the prophet had bid you do some great thing, would you not have
done it ?
something that was hard and difficult to done, or painful to bear, to go through some severe operation, or disagreeable course of physic:

how much rather then when he says to you, wash, and be clean ?
which is so easy to be done; though Abarbinel observes it may be interpreted, the prophet has bid you do a great thing, and which is wonderful; for though he has said, wash and be clean, consider it a great thing, and which is a wonderful mystery, and therefore do not despise his cure.