Robert Rauschenberg, Erased De Kooning Drawing (1953). In SFMOMA's collection, here. And some discussion of it at the Tate, here: "It was a performative act - the erasing is the important part of it - resulting in a conceptual work (you have to know that there was an actual de Kooning that was erased, with the artist's consent, to have full understanding of it)."