When did he say that?
Senator Obama said it to Nicholas Kristof of the New York Times; Mr. Kristof reported it on March 6, 2007 in his opinion column at http://select.nytimes.com/2007/03/06/opinion/06kristof.html?_r=1:
Mr. Obama recalled the opening lines of the Arabic call to prayer, reciting them with a first-rate accent. In a remark that seemed delightfully uncalculated (it’ll give Alabama voters heart attacks), Mr. Obama described the call to prayer as “one of the prettiest sounds on Earth at sunset.”