And the answer is….
Las Siete Partidas, passed by Alfonso X, El Sabio (1265) [Alfonso "The Wise"].
Congrats to Tatiana Siakka, David Mamane, Andrey, and Lorenzo Climenti! (Nico: you can afford 4 beers, right?)
Here is an explanation extracted from their answers:
Title 7 within Law 2 of the Fifth Partida, entitled “Of the shortages and bids that merchants create between themselves through oats and guilds” was the legal provision prohibiting traders from engaging in price-fixing and output restriction.
The Code was elaborated in Spain (Castile), but it was in force in Latin America until the modern codification movement (1822–1916). Until the beginning of the 19th century, they were even in effect in the parts of the United States, such as Louisiana, California or Nevada, that had previously belonged to the Spanish empire and used civil law. Furthermore, they served as the legal foundation for the formation of the governing juntas that were established in both Spain and Spanish America after the imprisonment of King Fernando VII during the Peninsular War.
Below you will find a scanned version of the relevant part by courtesy of José Luis Buendía.
[The text appears in Spanish and Latin. Since the short bios available at Brussels-based law firms suggest that all competition lawyers are fluent in practically every language, we trust that many of you will be able to understand it ;) ]
P.S. Could someone please edit wikipedia´s entry for History of Competition Law?