Earliest source located is the book Brighter than a Thousand Suns: A Personal History of the Atomic Scientists by Robert Jungk (1958), p. 249, which says that Einstein made the comment during "a walk with Ernst Straus, a young mathematician acting as his scientific assistant at Princeton."
Variant: "Equations are more important to me, because politics is for the present, but an equation is something for eternity." From A Briefer History of Time by Stephen Hawking (2005), p. 144 http://books.google.com/books?id=4Y0ZBW19n_YC&lpg=PP1&pg=PA144#v=onepage&q&f=false.
Earlier, Straus recalled the German version of the quote in Helle Zeit, Dunkle Zeit: In Memoriam Albert Einstein (1956) edited by Carl Seelig<!-- Zurich: Europa Verlag -->, p. 71. There the quote was given as Ja, so muß man seine Zeit zwischen der Politik und unseren Gleichungen teilen. Aber unsere Gleichungen sind mir doch viel wichtiger; denn die Politik ist für die Gegenwart da, aber solch eine Gleichung is etwas für die Ewigkeit.
Attributed in posthumous publications
Context: Yes, we now have to divide up our time like that, between politics and our equations. But to me our equations are far more important, for politics are only a matter of present concern. A mathematical equation stands forever.