Decree On Holy Orders

Letter 4,11 - to Maximian, bishop of Syracuse. September 593

" You should not allow priests, deacons and other clerics, who serve the churches, to become abbots in the monasteries. Rather, they should either give up their clerical service and be called to monastic orders, or if they have decided to remain in the role of abbot, they should in no way be permitted to continue in the service of a cleric.
For it is quite improper if someone cannot fulfill just one of these roles diligently, in proportion to its importance, and yet is judged suitable for both of them.
Thus the ecclesiastical order would impede the monastic life, and in turn the rule of monasticism would also impede the ecclesiastical benefits."

Cited from: The Letters of Gregory the Great, trans. John R.C. Martyn (Toronto: PIMS, 2004), I: 295