id est: expositio eucharistiae negocii ad Martinum Lutherum, by Huldrych Zwingli, was published on the 28th of February, 1527. It was the last time there was any semblance of friendliness between the two. The breach was insurmountable, in spite of later efforts to overcome the gulf.
You can read the whole book here. In it, Zwingli shows the proper understanding of the Lord’s Supper in opposition to the magical view of the Catholics and the ridiculous view of the Lutherans.
Zwingli’s closing salvo is fantastic- writing in conclusion to Luther that the Supper is ‘sign’, and absolving him (for his sinful misrepresentation of the supper!).
Hoc est externums signum, quod mentem certam reddit, non carmen: “Ego te absolvo”.
Luther’s reaction was – naturally – quite negative and downright hostile. Apparently he didn’t want to be forgiven his error.