If you are writing a formal business letter in which you need to cite other people's work, it is best to do so in a structured, formal manner that leaves no room for misinterpretation. You need to decide whether you are going to attribute the work you cite to a specific person, or simply to emphasize that it is not your work. Whatever the case, you must avoid plagiarism, the great taboo of any form of writing.
Write the letter with the purpose of being transparent from the start. If you intend to cite more than one person's work, it is preferable to start by mentioning you are going to do so. At that point you can point out whether you are going to name the person associated with the work in the body of the letter, or at the end in a source format.
Associate the author and her work clearly, so there is no misunderstanding about your reference. Word structure is particularly important here, as a clumsy sentence, or a comma in the wrong place, could confuse the reader. For example, if you are citing the work of Professor X, structure your sentence in the following way: ''The academic analysis needs to be analyzed by at least three experts, as pointed out in a study by Professor X.''
State the association of the other person's work so the reader has a clear understanding of the material being cited. If the work you are quoting comes from a study, a book or a website, make this clear by mentioning the title and, if possible, the date of publication. But be careful of providing too much information, as this can interrupt the flow of the letter and cause confusion.
Create a source reference at the bottom of the letter, or on a separate page, if you are citing several people's work. Place a number in superscript or in brackets after the citation. With respect to the earlier example, your citation would look like this: ''The academic analysis needs to be analyzed by at least three experts (1).'' At the end of the letter, or on the separate appendix, Professor X's name, as well as the publication from which you used his source, will appear next to 1, which should appear without brackets.
Check your letter thoroughly for any hint of plagiarism. Even if you have correctly cited work that is not your own, you may inadvertently have left other references unattributed or given the wrong impression that they are your ideas. If this is the case, and the recipient becomes aware of this, it will throw the basis of your letter into doubt, regardless of your intentions.