"Exact origins of the term are not clear. Its first identified appearance was in a secret telegram sent to Berlin by the German charge d’affaires in Alicante, Hans-Hermann Völckers dated September 30, 1936. He referred to an unidentified "supposed statement by Franco"...

...which "is being circulated" (apparently in the Republican zone or in the Republican-held Levantine zone). In the statement, Franco allegedly claimed that there were four Nationalist columns approaching Madrid and the fifth column waited to rise from the inside."

Militant supporters within the capital is the “fifth column,” intent on undermining the loyalist government from within.

"Emilio Mola Vidal, a Nationalist general during the Spanish Civil War (1936–39) has four of his army columns moved on Madrid, the general referred to his militant supporters within the capital as his “fifth column,” intent on undermining the loyalist government from within."

Treason and Sedition.