Canon 4
Let no one deceive himself, nor put forward the pretext of having found such property. For it is not lawful, even for a man who has found anything, to aggrandize himself by it. For Deuteronomy says: “You shall not see your brother's ox or his sheep go astray in the way, and pay no heed to them; but you shall in any wise bring them again unto your brother. And if your brother come not near you, or if you know him not, then you shall bring them together, and they shall be with you until your brother seek after them, and you shall restore them to him again.
And in like manner shall you do with his ass, and so shall you do with his raiment, and so shall you do with all lost things of your brother's, which he has lost, and you may find.” Thus much in Deuteronomy. And in the book of Exodus it is said, with reference not only to the case of finding what is a friend's, but also of finding what is an enemy's: “You shall surely bring them back to the house of their master again.” And if it is not lawful to aggrandize oneself at the expense of another, whether he be brother or enemy, even in the time of peace, when he is living at his ease and delicately, and without concern as to his property, how much more must it be the case when one is met by adversity, and is fleeing from his enemies, and has had to abandon his possessions by force of circumstances!
Source: Canonical Epistle (New Advent)