I was sent a link to a website "exposing" Calvinism because it contained a bunch of Luther quotes. Many of the quotes I've gone over already, but there were a few that I've never examined. Here's one in particular found in a pdf e-book (pictured left) from this site:
“Whoever rejects my doctrine cannot be saved.” 
 History of the German People at the Close of the Middle Ages - Page 96 by Johannes Janssen, A. M
The documentation "History of the German People at the Close of the Middle Ages - Page 96 by Johannes Janssen, A. M" is a reference to a work by a 19th Century Roman Catholic historian. Janssesn can be classified with the genre of anti-Luther Roman Catholic writers (a genre that is no longer the opinion of modern Roman Catholic scholarship). The page in question can be found here. Janssen states,
Luther's conviction that he was called by God to proclaim anew the fundamental truths of Christianity, which had been falsified and distorted since the days of the Apostles, led him to declare that he would have his teaching amended by no one, not even by angels. Whoever rejects my doctrine,' he said, ' cannot be saved.' It also led him to the opinion, long held by the Hussites and other heretical teachers of the fifteenth century, that the Pope was Antichrist, and that the Church was languishing in Babylonish captivity. And these two fixed ideas that he was a divinely inspired teacher and that the Pope was Antichrist dominated his whole life and work.This particular section lacks documentation, though this paragraph is placed in the context of the year 1518. The website using this quote probably didn't get the quote from Janssen, but rather Peter Wiener's Martin Luther, Hitler's Spiritual Ancestor. Wiener states (also without proper documentation):
“When I am angry, I am not expressing my own wrath, but the wrath of God”. Luther knew that he was superior to any man or saint. “St. Augustine or St. Ambrosius cannot be compared with me.” “They shall respect our teaching which is the word of God, spoken by the Holy Ghost, through our lips”. “Not for a thousand years has God bestowed such great gifts on any bishop as He as on me” (E61, 422). “God has appointed me for the whole German land, and I boldly vouch and declare that when you obey me you are without a doubt obeying not me but Christ” (W15, 27). “Whoever obeys me not, despises not me but Christ.” “I believe that we are the last trump that sounds before Christ is coming”. “What I teach and write remains true even though the whole world should fall to pieces over it.” (W18, 401). “Whoever rejects my doctrine cannot be saved.” “Nobody should rise up against me.”Without any sort of helpful reference or meaningful clues from these writers, one can only speculate as to the correct primary source. It's quite possible the source is Against the Spiritual Estate of the Pope and the Bishops Falsely So-Called, July 1522. There Luther says something almost exact to the quote in question. This writing is highly polemical. The editors of Luther's Works describe it as Luther's "...own 'bull' against the Roman heresy, he laid bare the many weaknesses and immoralities of the spiritual estate such as celibacy, political tyranny, and financial greed" (LW 39: Introduction). The issue provoking this treatise was renewed indulgence traffic. The editors of the treatise state, "Luther’s highly polemical and satirical language, more evident in this treatise than in others, was prompted by the recurrence of the indulgence traffic in the territory of Albrecht of Mainz" (LW 39:244). In particular:
On September 15, 1521, Archbishop Albrecht of Mainz celebrated the annual festival of relics at his newly built cathedral, the Church of St. Moritz and Mary Magdalene in Halle, with the official announcement that indulgences would be granted to all visitors to the exhibition of relics. Anyone who prayed at a shrine and gave alms was promised an indulgence of four thousand years; anyone who confessed his sins to one of the priests hearing confessions in the cathedral during the ten days of the celebration would receive a plenary indulgence. Pope Leo X had issued a bull in 1519 granting the cathedral of Halle the same privileges granted to the Church of St. Peter in Rome: its confessors were authorized to absolve cases usually absolved only by the apostolic see in Rome; in addition, they could convert vows into financial contributions for the completion of the Halle cathedral—privileges not unusual in the Luther, M. (1999, c1970). Vol. 39: Luther's works, vol. 39 : Church and Ministry I (J. J. Pelikan, H. C. Oswald and H. T. Lehmann, Ed.). Luther's Works (39:241). Philadelphia: Fortress Press.Keeping this background in mind, the obscure Luther quote in question comes from the very beginning of this treatise. If the "gospel" the romanists were putting forth was fraught with indulgence traffic, one wonders really how far off base Luther was from his polemical comment, "whoever does not accept my teaching may not be saved."
Martin Luther, ecclesiastic in Wittenberg by the grace of God: To the papal bishops [I offer] my service and self-understanding in Christ.
Although I might be regarded as a fool by you, dear lords, because of the haughty title I call myself, an ecclesiastic by the grace of God, you should know that I am not at all surprised by this. You curse, slander, condemn, persecute, and possibly even burn me as a heretic for the sake of a high and noble cause. In this you act as you please, according to the pleasure of your idol. As a result of God’s disfavor you have the virtue that you do not want to listen. Neither do you want to give an answer. Instead, like the hardened Jews you blasphemously and stubbornly want to condemn me without a hearing, without investigating the cause, without overcoming me. You are not even ashamed of letting a man defy you so frequently with such good reason. Very well then, since it is a question of lowering the horns and acting with brute force, I too have to lower my horns and risk my head for my Lord. In order to get things started, I call myself an ecclesiastic by the grace of God in defiance of you and the devil, although you call me a heretic with an abundance of slander. And even if I called myself an evangelist by the grace of God, I would still be more confident of proving it than that any one of you could prove his episcopal title or name. I am certain that Christ himself, who is the master of my teaching, gives me this title and regards me as one. Moreover, he will be my witness on the Last Day that it is not my pure gospel but his. Thus your raging and raving is not going to help you at all. Rather, the more you rage and rave, the haughtier we shall be toward you, with God’s help, and [the more we] shall despise your disgrace. Even though you might take my life, since you are murderers, you will annihilate neither my name nor my teaching. For you too will have to die at last and put an end to murder.
Now that I am deprived of my titles through papal and imperial disfavor and my bestial character is washed away with so many bulls that I need never be called either Doctor of Holy Scripture or some kind of papal creature, I am almost as shocked as an ass who has lost its bag. For these masks were my greatest shame before God. I too was once in error (which I learned from your crowd at great price and with great effort), a liar, a cheater, a seducer, and a blasphemer against God’s pure teaching, as you are now. But the Father of all mercy did not look at my vice, blasphemy, and my very sinful, evil life; instead, out of the infinite richness of his grace, he permitted me to know his Son, Jesus Christ, and to teach [him] to others, until we were certain of his truth. However, I need not have any title and name to praise highly the word, office, and work which I have from God and which you blind blasphemers defile and persecute beyond measure. I trust my praise will overcome your defiling, just as my justice will overcome your injustice. It does not matter if, with your blasphemy, you are on top for the moment.
Therefore, I now let you know that from now on I shall no longer do you the honor of allowing you—or even an angel from heaven—to judge my teaching or to examine it. For there has been enough foolish humility now for the third time at Worms, and it has not helped. Instead, I shall let myself be heard and, as St. Peter teaches, give an explanation and defense of my teaching to all the world [I Pet. 3:15]. I shall not have it judged by any man, not even by any angel. For since I am certain of it, I shall be your judge and even the angels’ judge through this teaching (as St. Paul says [I Cor. 6:3]) so that whoever does not accept my teaching may not be saved—for it is God’s and not mine. Therefore, my judgment is also not mine but God’s.
Finally, dear lords, let this be the conclusion: If I live you shall have no peace from me, and if you kill me you shall have ten times less peace, for I shall be, as Hosea says, a bear on the road and a lion in the street [Hos. 13:8]. No matter how you handle me, you shall not have your will until your iron head and stiff neck are broken with either grace or disgrace. If you do not improve as I would like to see you do, then it is agreed that you threaten with hostility and I do not care. May God grant that you know yourselves. Amen. Luther, M. (1999, c1970). Vol. 39: Luther's works, vol. 39 : Church and Ministry I (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther's Works (39:247). Philadelphia: Fortress Press.