Source, in full
Disparaging the work of Joseph Urban, his brother Addison's architectural rival. Harry Thaw was a wealthy man of the times who had shot and killed architect Stanford White over his earlier involvement with Thaw's wife Evelyn Nesbit.
Notes from Wikiquote
Quoted by Alva Johnston, The Legendary Mizners, 1953, Farrar Straus and Young, New York. Johnston allows that the quote has been attributed to many others, but makes a good case that Mizner said it first.