In what paper did Einstein posit the cosmological constant? Einstein's seminal papers On the Electrodynamics of Moving Bodies and Explanation of the perihelion movement of Mercury from general relativity were fundamental in the descriptions of Special and General Relativity.
In what paper did he posit the existence of a cosmological constant, in order to counteract the universal contraction that GR implies?
 A: The title of the paper in German, in which Einstein posits the existence of a cosmological constant, is "Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie", and its translation in English is: "Cosmological Considerations in the General Theory of Relativity". This paper was first published in 1917, in Sitzungsberichte der Preussischen Akad d. Wissenschaften$^1$ (Berlin) (1917) 142-152, in order to find a static universe model by use of the gravitational field equations of general relativity. This paper actually was a report to the Prussian Academy of Sciences (the same place where Albert Einstein presented his field equations of general relativity on 25 November 1915). In Einstein's paper, we see in German (Deutsch):

In this excellent review [2] (open access), you can find translations of many (almost all) parts of Einstein's article into English along with the historical developments of the Einstein's idea. In page 21 of this review, you can see the famous Einstein's field equations with a cosmological constant, the same as Eq. (13a). In Einstein's paper we read (translated in English by the authors of Ref. [2])

However, the system of equations (13) allows a readily suggested extension which is compatible with the relativity postulate, and is perfectly analogous to the extension of Poisson’s equation given by equation (2). For on the left-hand side of field equation (13) we may add the fundamental tensor $g_{μν}$, multiplied by a universal constant, $−λ$, at present unknown, without destroying the general covariance. In place of field equation (13) we write $${G_{\mu \nu }} - \lambda {g_{\mu \nu }} = -\kappa \left( {{T_{\mu \nu }} - \frac{1}{2}{g_{\mu \nu }}T} \right) \tag{13a}$$

You can also find the Einstein's original paper in this free book, pages 130-139, which is a collection of papers by Hendrik A. Lorentz, Albert Einstein and Hermann Minkowski (this book was first published in 1922). But, this book is in German and, eventually, I could find an English translation of that book including the Einstein's paper in this valuable, free book [4] (pages 177-188) which is a collection of original papers by Hendrik A. Lorentz, Albert Einstein, Hermann Minkowski and also Hermann Weyl [4]. However, as I said before, in Ref. [2], you can find translations of many (almost all) parts of Einstein's article into English along with the historical developments of the Einstein's idea including Einstein's manuscripts and his correspondence with E. Schrödinger and other physicists.

$^1$ Meeting reports of the Prussian Academy of Sciences



References
[1] A. Einstein, Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie, Sitzungsberichte der Preussischen Akad d. Wissenschaften$^1$ (Berlin) (1917) 142-152.
[2] C. O’Raifeartaigh, M. O’Keeffe, W. Nahm, and S. Mitton, Einstein’s 1917 static model of the universe: a centennial review, The European Physical Journal H 42 (2017) 431.
[3] H.A. Lorentz, A. Einstein, H. Minkowski, Das Relativitätsprinzip (The principle of relativity), Leipzig (1922), pages 130-139.
[4] H.A. Lorentz, A. Einstein, H. Minkowski, and with notes by A. Sommerfeld, The principle of relativity: a collection of original memoirs on the special and general theory of relativity, Courier Corporation (1952), pages 177-188.
