Attributed in "Mathematics as grammar: 'Grammar' in Wittgenstein's philosophy of mathematics during the Middle Period" by A. A. B. Aspeitia, Indiana University, (p. 25), 2000.

Gottlob Frege, J. L. Austin (1980). “The Foundations of Arithmetic: A Logico-Mathematical Enquiry Into the Concept of Number”, p.4, Northwestern University Press

Note in the appendix of "Grundlagen der Arithmetik" (Vol. 2), after Frege had received a letter of Bertrand Russell in which Russell had explained his discovery of, what is now known as, Russell's paradox, 1903.