More on tests

On Tuesday I posted this piece on being tested by God, in the context of Avot 5:4 which teaches about God setting Abraham ten tests, all of which he successfully negotiated. Since then I have been musing about a quotation that has floated past my line of vision many times in recent years. It runs along the lines of "God only sends a person a test He knows he can handle". This quote comes in various forms, but is never accompanied by any source.

Does anyone know who first expressed this sentiment? If so, please get in touch! It would be good to know. It would be even better to know if its originator had any empirical evidence on which to base it.