De constitutionibus principum
Digestum 1.4.1: 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 and to him its authority and power with the "royal law (lex regia)," which has been promulgated through his authority.
Codex 1.14.4: Digna vox maiestate regnantis legibus alligatm si principem profiteri; adeo de auctoritate iuris nostra pendet auctoritatem.
It is a worthy statement of the majesty of a reigning prince for him to profess to be subject to the laws; for our authority is dependent upon authority of the law.