Church History by Eusebius Pamphilius
1. But when he saw that he had not time for the deeper study of divine things, and for the investigation and interpretation of the Sacred Scriptures, and also for the instruction of those who came to him, -- for coming, one after another, from morning till evening to be taught by him, they scarcely gave him time to breathe, -- he divided the multitude. And from those whom he knew well, he selected Heraclas, who was a zealous student of divine things, and in other respects a very learned man, not ignorant of philosophy, and made him his associate in the work of instruction. He entrusted to him the elementary training of beginners, but reserved for himself the teaching of those who were farther advanced.