De constitutionibus principum

Ulpianus libro primo institutionum.  Quod principi placuit legis habet vigorem, utpote cum lege regia quae de imperio eius lata est, populus ei et in eum omne suum imperium et potestatem conferat.  Quodcumque igitur imperator per (ante correctionem) conferat

What is pleasing to the prince has the force of law.  A decision given by the emperor has the force of a statute.  This is because the people bestows on him its authority and power with the "royal law (lex regia)."

Digestum 1.4.1