TITLE: Origin of Hecke operators
QUESTION [5 upvotes]: What is the original paper in which Erich Hecke had first introduced the Hecke operators?

REPLY [16 votes]: Über Modulfunktionen und die Dirichletschen Reihen mit Eulerscher Produktentwicklung. I, Math. Ann. 114 (1937), 1-28; II, ibid., 316-351. These two papers are available here and here.