Sometimes it happens that while writing an answer, we get some help from other answers posted on AskUbuntu.
In such cases, should we cite the sources?
If you're pulling in an answer from another question on Ask Ubuntu, you should ask yourself whether this question should actually just be closed as a duplicate.
But assuming it's not a duplicate:
Citations are required for any content you take from Ask Ubuntu (or other SE sites) so please don't omit them if you're copying portions of answer around.
And otherwise, why wouldn't you? A citation or source to something that explains what you're talking about only improves your answer. Include it.
If you're not copying any part of your source (that is, not verbatim, but also not even in modified form), then a citation is unlikely to be required by copyright law.
But you should still have it. Explaining where you got some of the information in your answer only improves your answer (as Oli says), even in situations where you are not copying anything from your source.