Duns Scotus, Rep. IA d. 11 q. 2 (Wolter-Bychkov I, 420):
Idem enim est quiditas et esse.
Here it is in context:
To the first reason, when it says that relations would distinguish either according to quiddity or according to being, I do not understand nor see what philosophy asserts this. For quiddity and being is the same thing. Hence, each distinguishes, because the being of a relation is 'towards another' just as its quiddity is, for the same thing both remains and passes over [into the essence], both according to being and according to quiddity. What, then, is [this relation] formally? Remaining as 'being towards another'. For it is not towards, or in relation to itself, as Augustine says: "What makes him God is not the same as what makes him Father." At the same time, to pass over is for something to be really the same thing as something else, not to form a composition with the latter. And it is in this way that a relation in the divine passes into [essence], because [then] it is the same thing as the essence, not producing a composition with it. Hence, the aforesaid distinction of a relation is non-existent; for this is to distinguish something into two relationships, one of which is nothing. For a relation compared to its foundation is a nothing, because then it is not a relation, but only in potency towards an opposite. Hence, a relation in every way in which it is a relation is 'towards another' and is some reality; however according to how one thinks of it as compared to the foundation, it is the mean, as it were, as it is compared to the term.