Subgroup of Mobius Transformations that are isometries

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How would I go about finding the subgroup of all Mobius transformations such that they are all isometries with respect to the study metric? I have started off by showing the restrictions required for a subgroup preserving the upper half plane but not sure how much this will help. I've read that the answer is just the $SU(2)$ group of matrices but can't find why this is the case.