The Einstein 1916 Annalen der Physik paper is called "Die Grundlage
der allgemeinen Relativitatstheorie", and appears translated as "The
Foundations of the General Theory of Relativity" in the collection of
papers:
Einstein, A., Lorentz, H.A. Weyl, H. Minkowski, H. 1923. _The
Principle of Relativity_. Translated by W. Perrett and G.B. Jeffery,
with notes by A Sommerfield. Dover Books, 1952.
This is from Einstein's 1916 paper, (Sc 5 "Contravariant and Covariant
Four-Vectors", p. 122):
Note on a Simplified Way of Writing the Expressions.
---A glance at the equations of this paragraph shows
that there is always a summation with respect to the
indices which occur twice under a sign of summation
(e.g., the index \vu in (5)), and only with respect to the
indices which occur twice. It is therefore possible,
without loss of clearness, to omit the sign of summation.
In its place we introduce the convention:---If an index
occurs twice in one term of an expression, it is always
to be summed unless the contrary is expressly stated.
(Einstein 1916 (1952), p. 122)