Must answers provide citations if using external sources, and if so, how should the citing be done?

Question

I've received an answer where at least 2 sentences were taken from an article on another site and the site was not credited in the answer. Much of the rest may or may not have been from the article. The article was summarized. I had flagged the answer for moderation and while the author did add a link on his own, the answer has since been deleted.

I have, myself, answered two questions where, prior to summarizing articles on other websites, I've linked to the article as Article Title (Website Name). The only time I've used text directly from the article, I make it obvious it is a direct quote and credit the original author of the text used. I have also used my own text.

I would link to the aforementioned answers, but I don't want to identify the one not answered by me (and it's been deleted anyway), or unjustly promote my own answers (which you can find on your own through my profile if you needed to).

What's the proper procedure?

Answer 1

Citing without proper attribution is definitely a no-no in my book. StackExchange is licensed under an attribution-share alike creative commons licence. Not to cite your sources is really bad behaviour.

In general, though, I would always prefer an answer that cites and links to credible sources rather than what is otherwise just someone's opinion. As such, while I wouldn't set a requirement for citing supporting evidence (we are not Wikipedia and anecdotal evidence is very useful to people as well), I would very much encourage it by upvoting and accepting answers.