This is interesting. An atheist discussion of ad hominem attack recognizes the validity of questioning credibility of a person making a claim as not an ad hominem argument, which, of course, is very different from rejecting a logical conclusion based on a person's own actions or associations with persons of ill repute.

It seems, then, that atheists would recognize that a demand to supply sources is legitimately subject to a criticism of the credibility of that person's own sources, and such a criticism does not constitute ad hominem attack.