"The earliest recognition of the observation of Sunday as a legal duty is a constitution of Constantine in 321 A.D. enacting that all courts of justice, inhabitants of towns, and workshops were to be at rest on Sunday (venerabili die Solis), with an exception in favor of those engaged in agricultural labor." Encyclopedia Britannica, ninth edition, article "Sunday." 

The Latin original is in the Codes Justiniani (Codes of Justinian), lib. 3, title 12, lex. 3. 

The law is given in Latin and in English in Philip Schaff's History of the Christian Church, Vol. 3, 3d period, chapter 7, sec. 75, pg. 380, footnote 1. 

And in Albert Henry Newman's A Manual of Church History, (Philadelphia: The American Baptist Publication Society, 1933),rev.ed., Vol. 1,pp. 305-307. 

And in Leroy E. Froom's, The Prophetic Faith of Our Fathers (Washington, D.C.: Review & Herald Publishing Asoc., 1950), Vol. 1, pp. 376-381.