Van der Waerden's conjecture (now a theorem of Egorychev and Falikman) states that the permanent of a doubly stochastic matrix is at least $n!/n^n$.
The Wikipedia article, as well as many other sources, provides the following citation:
B. L. van der Waerden, "Aufgabe 45", Jber. Deutsch. Math.-Verein., 35 (1926), 117.
However, I just went to the library to look up volume 35 of Jahresbericht der Deutschen Mathematiker-Vereinigung, and page 117 lies right smack in the middle of a paper by Karl Menger, "Bericht über die Dimensionstheorie." There is nothing remotely resembling Van der Waerden's conjecture on page 117.
In the book A Course in Combinatorics by J. H. van Lint and R. M. Wilson, there is a chapter on Van der Waerden's conjecture, and there is a footnote saying that Van der Waerden told one of the authors that he had never made any such conjecture.
So maybe Van der Waerden never made the conjecture? Or did he make the conjecture and then forget about it, with the citation getting garbled somewhere along the line? The earliest citation to "Aufgabe 45" in Google Scholar seems to be from 1960, so maybe that's when the error was introduced. But I don't know how to search for earlier citations to a possibly non-existent reference by Van der Waerden.
ADDENDUM (prompted by Mark Sapir's comments to Matt F.'s answer below): Here's what Van Lint and Wilson say (Chapter 12, 2nd edition):
In 1926, B. L. van der Waerden proposed as a problem to determine the minimal permanent among all doubly stochastic matrices. It was natural to assume that this minimum is per $J_n = n!\,n^{-n}$. … The assertion [that this minimum is uniquely achieved by the constant matrix] became known as the ‘Van der Waerden conjecture’ (although in 1969 he told one of the present authors that he had not heard this name before and that he had made no such conjecture).
So it sounds like Van der Waerden himself saw a distinction between asking for the minimum value, and conjecturing what the minimum value is (or conjecturing that the minimum value is uniquely attained by the constant matrix).
Best Answer
Aufgabe 45 is in the second Abteilung of volume 35, indeed on page 117, as here. He was asking about permanents, but rather than asking for a proof about $n!/n^n$, he asked for the determination of a minimum.
Translation: "The Function $Q\cdots$ (summing over all permutations of the indices $1,\ldots,n$), under the conditions $a_{ik}\cdots$, can only take positive values (Dénes König, Graphs and their Applications #2, Math. Ann. 77, p. 47). The minimum of the function is (under the given conditions) to be determined."