John Cassian, The Institutes IV.13: How wrong it is considered for any one to say that anything, however trifling, is his own

Among their other practices I fancy that it is unnecessary even to mention this virtue, viz., that no one is allowed to possess a box or basket as his special property, nor any such thing which he could keep as his own and secure with his own seal, as we are well aware that they are in all respects stripped so bare that they have nothing whatever except their shirt, cloak, shoes, sheepskin, and rush mat; for in other monasteries as well, where some indulgence and relaxation is granted, we see that this rule is still most strictly kept, so that no one ventures to say even in word that anything is his own: and it is a great offence if there drops from the mouth of a monk such an expression as “my book,” “my tablets,” “my pen,” “my coat,” or “my shoes;” and for this he would have to make satisfaction by a proper penance, if by accident some such expression escaped his lips through thoughtlessness or ignorance.