WP:Verifiability

In Handwiki, verifiability means that people can check that facts or claims correspond to reliable sources. Content is determined by published information rather than editors' beliefs, experiences, or previously unpublished ideas or information. Even if you are sure something is true, it must have been published in a reliable source before you can add it. If reliable sources disagree with each other, then maintain a neutral point of view and present what the various sources say, giving each side its due weight.

Each fact or claim in an article must be verifiable. All quotations, and any material whose verifiability has been challenged or is likely to be challenged, must include an inline citation to a reliable source that directly supports the material. Any material that needs an inline citation but does not have one may be removed. Please immediately remove contentious material about living people that is unsourced or poorly sourced.

When material that needs an inline citation appears in two or more articles, an inline citation is needed in each.

For how to write citations, see citing sources. Verifiability, no original research, and neutral point of view are Wikipedia's core content policies. They work together to determine content, so editors should understand the key points of all three. Articles must also comply with the copyright policy.