Res Gestae

But when he came to the Forum of Trajan, a construction unique under the heavens, as we believe, and admirable even in the unanimous opinion of the gods, he stood fast in amazement, turning his attention to the gigantic complex about him, beggaring description and never again to be imitated by mortal men. Therefore abandoning all hope of attempting anything like it, he said that he would and could copy Trajan’s steed alone, which stands in the centre of the vestibule, carrying the emperor himself.