If the critical distinguishing feature of the formulation "A great man once said [or wrote]" is the ascribing of an actual quotation—and not merely an aphorism—to an unnamed "great man," perhaps an instance from Montaigne's 1588 essay "De l'Art de Conferer" ("Of the Art of Conferring [or Conference]" in earlier translations; "Of the Art of Discussion" in Donald M. Frame's translation) qualifies, though Montaigne may have expected his readers to recognize the quotation and to know that its author was Quintus Curtius:
Ce qu'il [Tacitus] dict aussi que Vespasian, par la faveur du Dieu Serapis, guarit en Alexandrie une femme aveugle en luy oignant les yeux de sa salive, et je ne sçay quel autre miracle, il le faict par l'exemple et devoir de tous bons historiens: ils tiennent registre des evenements d'importance; parmy les accidens publics sont aussi les bruits et opinions populaires. C'est leur rolle de reciter les communes creances, non pas de les regler. Cette part touche les Theologiens et les philosophes directeurs des consciences. Pourtant tres-sagement, ce sien compaignon et grand homme comme luy: Equidem plura transcribo quam credo: nam nec affirmare sustineo, de quibus dubito, nec subducere quae accepi;
From the John Florio translation (1603):
Where he [Tacitus] also saith that Vespasian, by the favour of the God Serapis, healed in the citie of Alexandria a blinde woman with the rubbing and anointing her eyes with fasting spettle, and some other miracles, which I remember not well now, he doth it by the example and devoire of all good historians. They keepe a register of important events; among publike accidents are allso popular reports and opinions. It is their part to relate common conceits, but not to sway them. This part belongeth to Divines and Philosophers, directors of consciences. Therefore, that companion of his, and as great a man as hee, said most wisely: Equidem plura transcribo quam credo: Nam nec affirmare sustineo de quibus dubito nec sub ducere quæ accepi: 'I write out more then I beleeve: for neither can I bide to affirm what I doubt of, nor to withdrawe what I have heard.'
Retranslated by Charles Cotton (in an edition published in 1877) as:
What also he [Tacitus] says, that Vespasian, "by the favour of the god Serapis, cured a blind woman at Alexandria by anointing her eyes with his spittle, and I know not what other miracle," he says by the example and duty of all his good historians. They record all events of importance; and amongst public incidents are the popular rumours and opinions. 'Tis their part to relate common beliefs, not to regulate them: that part concerns divines and philosophers, directors of consciences; and therefore it was that this companion of his, and a great man like himself, very wisely said: "Equidem plura transcribo, quam credo: nam nec affirmare sustineo, de quibus dubito, nec subducere quae accepi;"
Montaigne never cites Quintus Curtius by name in this essay or in any other; a scholarly work ("List of Some Authors Read by Montaigne" in Grace Norton, Studies in Montaigne (1904), page 279) asserts that Montaigne quotes Quintus Curtius seven times in the course of his Essays without attribution.