Link to the relevant section of Obama's "A World that Stands as One" speech in Berlin on 24th July 2008:
...which begins:
"That is why the greatest danger of all is to allow new walls to divide us from one another. The walls between old allies on either side of the Atlantic cannot stand. The walls between the countries with the most and those with the least cannot stand. The walls between races and tribes; natives and immigrants; Christian and Muslim and Jew cannot stand. These now are the walls we must tear down."