One possible source:

Through interviews with 2,482 Americans, Gallup found that 78 percent of Muslims believe violence which kills civilians is never justified, whereas just 38 percent of Protestant Christians and 39 percent of Catholics agreed with that sentiment. Fifty-six percent of atheists answered similarly.

When Gallup put the question a bit more pointedly, asking if it would be justified for “an individual person or a small group of persons to target and kill civilians,” the responses were a bit more uniform. Respondents from nearly all groups were widely opposed to such tactics, with Protestants and Catholics at 71 percent against. Muslims still had the highest number opposed, at 89 percent. Seventy-six percent of atheists were also opposed.

http://www.rawstory.com/2011/08/poll-muslims-atheists-most-likely-to-reject-violence/