You can buy LW 59 here):
"I know a great archbishop, whom I shall not name,* who had a high opinion of St. Cyprian, the holy bishop and martyr, and read a bit in his books [to arm himself] against the Lutherans, intending thereby entirely to overthrow them. But when it was pointed out to him that in the books of the same St. Cyprian it is written that the holy Christian Church is found not only in Rome but in every corner of the world, he said, 'If I had known that Cyprian taught that, I would have had his books burned as those of a heretic.' And when the passage in the book was shown to him,** he threw Cyprian and his book away and would no longer read the heretic."
*Probably Albert of Brandenburg, Archbishop of Mainz
**Cyprian of Carthage was renowned as a scourge of heretics, but he was equally renowned for his view that the apostolic authority conferred on Peter was shared equally by all bishops and that the bishop of Rome had no right to call himself "bishop of bishops" and exact obedience from all the rest. The passage Luther has in mind was probably On the Unity of the Church 4-5 (PL 4:499-502; ANF 5:422-23), but the point is made elsewhere as well (see, e.g., the acts of the Seventh Council of Carthage [PL 3:1053-54; ANF 5:565]).