When the Nazis came for the communists

I have often heard it quoted but not ever seen it sourced.

When the Nazis Came
by Martin Niemöller

When the Nazis came for the communists,
I was silent,
I was not a communist.

When they locked up the social democrats,
I was silent,
I was not a social democrat.

When they came for the trade unionists,
I was silent,
I was not a trade unionist.

When they came for me,
there was no more,
to speak out.