Only refer to published materials or materials that have been accepted for publication. This is necessary because only published materials are generally available for independent inspection. If you refer to unpublished materials, theses or abstracts, others will not necessarily be able to access them. Consequently, a part of your evidence will not be available for independent verification.

You must refer to the instructions to authors to make sure you do this as required by the journal. One of the most tedious aspects of writing for science is complying with the different styles used by different journals both for how literature is referred to in the text and how the papers are listed in the reference section.