Allegedly, "The history of Jonah is so monstrous that it is absolutely incredible ... " according to Luther. In this brief snippet, the implication may be that Luther denied the historical account of Jonah and the whale, and therefore denied the miraculous, and is also probably like one of those liberal scholars who interprets the Bible as fictional literature rather than history. Or quite possibly, he just said awful things about Holy Scripture.
This is from a Tabletalk entry, and not something Luther actually wrote. You can read the context for yourself. It does no harm to Luther.
The majesty of the prophet Jonah is surpassing. He has but four chapters, and yet he moved therewith the whole kingdom, so that in his weakness, he was justly a figure and a sign of the Lord Christ. Indeed, it is surprising, that Christ should recur to this but in four words. Moses likewise, in few words describes the creation, the history of Abraham, and other great mysteries; but he spends much time in describing the tent, the external sacrifices, the kidneys and so on; the reason is, he saw that the world greatly esteemed outward things, which they beheld with their carnal eyes, but. that which was spiritual, they soon forgot.
The history of the prophet Jonah is almost incredible, sounding more strange than any poet's fable; if it were not in the Bible, I should take it for a lie; for consider, how for the space of three days he was in the great belly of the whale, whereas in three hours he might have been digested and changed into the nature, flesh and blood of that monster; may not this be said, to live in the midst of death? In comparison of this miracle, the wonderful passage through the Red Sea was nothing
But what appears more strange is, that after he was delivered, he began to be angry, and to expostulate with the gracious God, touching a small matter not worth a straw. It is a great mystery. I am ashamed of my exposition upon this prophet, in that I so weakly touch the main point of this wonderful miracle. [source]