About Diocletian retiring to farm cabbages? Epitome de Caesaribus is one source.

5. Diocletian actually relinquished the imperial fasces of his own accord at Nicomedia and grew old on his private estates. 6. It was he who, when solicited by Herculius and Galerius for the purpose of resuming control, responded in this way, as though avoiding some kind of plague: “If you could see at Salonae the cabbages raised by our hands, you surely would never judge that a temptation.”