I believe the correct answer is: “Las Meninas” by Diego Velázquez.
John Singer Sargent's technique of applying paint in thin layers was influenced by the “Las Meninas”, a painting by Diego Velázquez, the leading artist of the Spanish Golden Age. Sargent painted a small-scale copy of the painting “Las Meninas”, after which he painted a homage to the painting in his “The Daughters of Edward Darley Boit”.