Since the linked proof was published in a mathematical journal [Bull. Amer. Math. Soc. Volume 53, Number 6 (1947), 509], I think it's safe to assume that Niven

Since the linked proof was published in a mathematical journal [Bull. Amer. Math. Soc. Volume 53, Number 6 (1947), 509], I think it's safe to assume that Niven was first.