Eutropius, Brevarium ab urbe condita

Book IX

§26 Diocletian was of a crafty disposition, with much sagacity, and keen penetration. He was willing to gratify his own disposition to cruelty in such a way as to throw the odium upon others; he was however a very active and able prince. He was the first that introduced into the Roman empire a ceremony suited rather to royal usages than to Roman liberty, giving orders that he should be adored, whereas all emperors before him were only saluted. He put ornaments of precious stones on his dress and shoes, when the imperial distinction had previously been only in the purple robe, the rest of the habit being the same as that of other men.

Sextus Aurelius Victor, Liber de caesaribus

§39 [H]e was, in fact, the first who really desired a supply of silk, purple, and gems for his sandals, together with a gold-brocaded robe. Although these things went beyond good taste and betrayed a vain and haughty disposition, they were nevertheless trivial in comparison with the rest. For he was the first of all after Caligula and Domitian to permit himself to be called “Lord” in public and to be worshiped and addressed as a god.

Ammianus Marcellinus, Res Gestae

Book XV

§5,18 Now it was the emperor Diocletian who was the first to introduce this foreign and royal form of adoration [kissing the purple], whereas we have read that always before our emperors were saluted like judges.