Source, in full
Used as an early slogan at Apple Computer in 1977, but the earliest occurrence of this exact phrase is found in the 1952 article Simplicity Marks New Dining Room by Elizabeth Hillyer. Earliest known attribution to Leonardo da Vinci is in Wisdom Through the Ages: Book Two (2003) by Helen Granat, p. 225, where it is attributed without any citation. The quotation "The height of sophistication is simplicity." is found in Clare Boothe Luce's Stuffed Shirts (1931)