I got this from an account of The Marburg Colloquy over on Begars All:
Luther: We decline naming them [the Fathers who taught the Real Presence]. Augustine wrote the passage you have quoted in his youth, it is moreover very unintelligible. Besides, I do not concern myself as to what the Fathers teach on this head, but I abide by the words of Christ, (Here he pointed again to the words written in chalk upon the table, "This is my body.") See, so they run. You have not driven us out of this stronghold, as you proudly imagined you would do, and we concern ourselves no farther about proofs.
This occurs near the end of the Colloquy, so very soon afterwards the two camps departed, never to be united in doctrine again. The section I cited above is Luther's final argument, basically, and he says he will stand by Christ's words "This is my body", no matter how many logical arguments Zwingli and co. can come up with. This is, I think, the essence of Lutheran Sola Scriptura, in that what God has expressly revealed, we are to believe because it comes from God, not because logical arguments can be deduced. It also ties in with the ongoing micro-blogstorm regarding Luther's alleged statement that councils would be required to judge between Luther's and Zwingli's divergent interpretation of the Scriptures--an argument made by some RC apologists for centuries. I am not a Luther scholar, but doesn't it seem a little odd that the guy who simply said "I abide by the words of Christ" would suddenly have a change of heart about councils? I am not saying it is impossible, just that the possibility is becoming more remote the more I learn about the events around that time.