This quote is attributed to Einstein.
Did he say it?
It was published as part of an interview with Einstein in the Saturday Evening Post of October 1929 by George Sylvester Viereck. Einstein did not write it himself.
The interviewer quoted him, and wrote it down in English. Einstein presumably said it in German (his command of German being far superior to his command of English). The source for the quote is the translated version in English.
The exact quotation in the original language has not been preserved. Here is the translation that was published by the interviewer, Viereck:
Viereck: "Do you look upon yourself as a German or as a Jew?"
Einstein: "It is quite possible, to be both. I look upon myself as a man. Nationalism is an infantile disease. It is the measles of mankind."
It may not satisfy you to learn that the quote is attributed to Einstein by the interviewer, rather than being written by Einstein himself. However, Einstein reiterates the mindset in "My Credo" (1932). He also recorded this statement. In it, he says:
I am against any nationalism, even in the guise of mere patriotism. Privileges based on position and property have always seemed to me unjust and pernicious, as did any exaggerated personality cult.
Smart man, that Einstein guy.