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.

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.

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