I can find a closer candidate from one his commentary on Psalm 130. He says that it is the doctrine with which the Church stands or falls. There appear to be two different texts, one labeled “Hs” and another “Dr.” I’m not sure what they mean, but Hs appears to be from a series of lectures perhaps while Dr is an uninterrupted work. Perhaps one of our Lutheran friends or someone familiar with Luther’s writings can explain. The following quotes can be found on WA 40, III, p. 351, line 34 and p. 352, line 3 respectively.
Hs] ‘Ut timearis’: Sic audivistis heri, quod iste versus sit Summa doctrinae Christianae et ille sol, qui illuminat Sanctam ecclesiam dei, quia isto articulo stante stat Ecclesia, ruente ruit Ecclesia. ‘That thou mayest be feared’: So you heard yesterday, because that verse is sum of Christian doctrine and the sun which illuminates the Holy Church of God, because when this article stands, the Church stands, and when it falls, the Church falls.]
Dr] Stante enim hac doctrina stat Ecclesia, ruente autem ruit ispa quoque. [If this doctrine stands, the Church stands, but if it falls, the Church also falls.]
archive.org/stream/werkekritischege4003luthuoft#page/352/mode/2up
Another example of a like-minded quote, if not as close in particular words, is from the Smalcald Articles.
**Part II, Article I: The first and chief article.
**
4] Now, since it is necessary to believe this, and it cannot be otherwise acquired or apprehended by any work, law, or merit, it is clear and certain that this faith alone justifies us as St. Paul says, Rom. 3:28:
For we conclude that a man is justified by faith, without the deeds of the Law. Likewise 3:26:
That He might be just, and the Justifier of him which believeth in Christ.
5] Of this article nothing can be yielded or surrendered [nor can anything be granted or permitted contrary to the same], even though heaven and earth, and whatever will not abide, should sink to ruin.
For there is none other name under heaven, given among men whereby we must be saved, says Peter, Acts 4:12.
And with His stripes we are healed, Is. 53:5.
And upon this article all things depend which we teach and practice in opposition to the Pope, the devil, and the [whole] world. Therefore, we must be sure concerning this doctrine, and not doubt; for otherwise all is lost, and the Pope and devil and all things gain the victory and suit over us.
bookofconcord.org/smalcald.php#mass