And what shall I say of the poets? Oh, this poor century of ours! In the coming year many of them will make their start, but not one of them is as bad as Cervantes, or idiotic enough to praise Don Quixote.
De poetas no digo: buen siglo es éste. Muchos están en ciernes para el año que viene; pero ninguno hay tan malo como Cervantes ni tan necio que alabe a don Quijote.
Original text (Spanish)
Source, in full
Letter dated August 14, 1604; cited from Nicolás Marín (ed.) Cartas (Madrid: Clásicos Castalia, 1985) p. 68. Translation by Ilsa Barea, from Sebastià Juan Arbó Cervantes: Adventurer, Idealist, and Destiny's Fool (London: Thames and Hudson, 1955) p. 204.