In Harper’s Weekly on December 20, 1913, Louis D. Brandeis wrote in an essay called “What Publicity Can Do”: “Sunlight is said to be the best of disinfectants; electric light the most efficient policeman.” The essay was reprinted in his 1914 book Other People’s Money.
Americans usually quote Brandeis as “Sunlight is the best disinfectant.” His words “is said to be” show that he was citing a saying. The short form had already appeared in print in 1879.