It wouldn't happen to be part of the dialogue in Jostein Gaarder's Sophie's World would it? I don't have a reference on hand, but there's a bit early in the novel where the philosopher is talking to Sophie about Aristotle and lego blocks - if I remember right - essentially dealing with the question of the Identity of Indiscernibles. In any case, if you read up around that problem - which seems to be what you're dealing with - you might find further clues.

Alternatively, it might be buried somewhere in Leibniz's many correspondences, but that's a pure guess on my part.

Droplets from a constant but intermittent dripping of water in a room with no wind would have no differences that we can discern with the naked eye. However, nothing is exactly the same on a molecular level.

The law of identity originates from classical antiquity. The modern formulation of identity is that of Gottfried Leibniz.

Ludwig Wittgenstein writes (Tractatus 5.5301):

"That identity is not a relation between objects is obvious."

At 5.5303 he elaborates:

"Roughly speaking: to say of two things that they are identical is nonsense,

"Roughly speaking: to say of two things that they are identical is nonsense,

I take this statement as distinct from the OP. The OP suggests that there is some variation in any two objects. I see that as an empirical statement.

Your quote points out the incoherence in suggesting that two things be one. That is, identity specifies a specific object as being that object, so you couldn't have two identical objects, if for no other reason than you've already defined them as two different things. I see that as a definitional/logical statement..

I'm thinking Loren Eiseley described Louis Agassiz's giving a student (who may have been named) a fish with the assignment to describe it. Somehow, Agassiz kept asking the student to do a better job. After weeks and many attempts, the fish being much the worse for wear, the student began to have some understanding of the fish. I do not have the reference in hand.

Or maybe Heraclitus's not being able to step into the same river twice, and Aristotle's joke that according to Cratylus, you couldn't even once.