From Tertullian's De Carne Christi (On the Flesh of Christ):
Crucifixus est dei filius; non pudet, quia pudendum est. Et mortuus est dei filius; credibile prorsus est, quia ineptum est. Et sepultus resurrexit; certum est, quia impossibile est.
Or in English:
The son of God was crucified; it is not silly, because it must be silly. And the son of God died; it is absolutely credible, because it is daft. And the buried rose again; it's certain, because it is impossible.
(Note that some sources appear to omit the final "est".)
These remarks of Tertullian are quoted out of context; he was not preaching irrationalism. Rather, he was attacking the idea that the resurrection was a confidence trick that was designed to dupe gullible people into believing in Christianity. He argued that a confidence trick would have to be believable; and the resurrection was so incredible no one would think it could fool anyone unless it was actually the truth.
This is however not a strong argument, as can be seen by David Hume's criticism of arguments based on the miraculous: see the miracles node for details. If we are faced with something impossible, it is more rational to assume we are mistaken or being deceived than to assume that something amazing and totally outside our bounds of experience has occurred.
Less sacred use was made of this Tertullian quotation in Edgar Allan Poe's story Berenice, where the resurrection of the title character is due to her premature burial when she falls into a deep epileptic trance. In this case, the resurrection is not impossible and is explainable through science; Poe uses the quote to add a religious mysticism to the events that unfold. His hero, full of grief at the illness and death of his beloved, is overtaken by a mental disorder:
My books, at this epoch, if they did not actually serve to irritate the disorder, partook, it will be perceived, largely, in their imaginative and inconsequential nature, of the characteristic qualities of the disorder itself. I well remember, among others, the treatise of the noble Italian, Coelius Secundus Curio, "De Amplitudine Beati Regni Dei;" St. Austin's [sic] great work, the "City of God;" and Tertullian's "De Carne Christi," in which the paradoxical sentence "Mortuus est Dei filius; credible est quia ineptum est: et sepultus resurrexit; certum est quia impossibile est," occupied my undivided time, for many weeks of laborious and fruitless investigation.
References:
http://www.geocities.com/latingreeksite/epigrams/tertullian.htm
www.tertullian.org/quotes.htm
And Everything2, of course (where you will find Berenice).