6.5.1 WHEN TOCITE
The first challenge for many legal writers is figuring out when citations are necessary. Generally, you must include a citation at the end of every sentence in which you state a legal proposition, refer to a new authority, or quote or paraphrase information from a court opinion (or other source).
One of the few occasions on which you may omit a citation is when you have already analyzed an authority and are applying that authority’s rule to your case. For example, the statement that “the McGuffin rule applies here” does not need a citation in a discussion in which the writer has already introduced and cited McGuffin.Some legal propositions are so basic that they may seem self-evident. If you are asking a court to apply such a legal principle to your client’s case, however, you should cite an authority that controls in that court’s jurisdiction.
BAD EXAMPLE It is well known that the Fifth Amendment’s protection against compelled self-incrimination applies to the states. 
GOOD EXAMPLE The Fifth Amendment’s protection against compelled self-incrimination applies to the states through the Fourteenth Amendment’s Due Process Clause. See Malloy v. Hogan, 378 U.S. 1, 6 (1964).
Legal writing is referenced writing, and readers expect frequent citation. You should use short citation forms and effective sentence structures to keep your writing readable, but you must include citations whenever you state a legal proposition, refer to an authority for the first time, or quote or paraphrase material from a source. This means that within the rule explanation section, for example, every sentence may have a citation after it. Many of the citations may be in the “Id.” form, but the reader will still expect and need citations.