Legal writing, in general, requires very precise and exhaustive use of citations. It is used to allow the reader to quickly go to a specific page of a specific source to find the support for whatever argument you are making.
As a basic rule, include a citation for anything you use from any type of authority. For legal memoranda and court documents you will want to include a citation for all facts, thoughts, or opinions that you gather from any outside source, even when you are not using direct quotations. These sources include:
EXCEPTION: You do not need a citation for something you have already previously discussed OR when you are discussing your own facts.