Chris Stinnett wrote an article eight years ago about attributing the source of illustrations in sermons. It is a particularly helpful article. But he ends the article by giving a checklist for verbal attributions:
"Not every illustration merits citation. So, when should we footnote material during sermon delivery?
When they said it better. There aren’t many new ideas under the sun, but many are better stated than ours. Borrowing a memorable line without attribution is plagiarism. By including the source, we show integrity and professionalism in research and presentation.
When expert opinion is helpful. We can lend power to the information by disclosing its origin and the authority behind it.
When it sounds too good to be true. Illustrations sometimes sound contrived and, therefore, manipulative. In the case of an amazing but true story, citing the source makes it more believable.
When local people are involved. Given opportunity to quote a hometown hero, the inclusion of the person’s name will stimulate interest in the rest of the presentation.
When the sermon is distributed in print. In such cases, full attribution is a must."