Look up the definition of the word "postulate." Be sure to choose the noun definition. According to the Merriam-Webster dictionary, postulate means "a hypothesis advanced as an essential presupposition, condition, or premise of a train of reasoning."
Find the original source of the postulate. If using Einstein's postulates that lead to the special theory of relativity, you must cite his 1905 paper "On the Electrodynamics of Moving Bodies." You should be aware of editions, different printings, translators, or any other factors indicating which version of the text you are citing.
Employ a signal phrase to indicate what you are quoting or paraphrasing is a postulate. If you want to cite Einstein's postulates that led to the special theory of relativity, it would appear as such: According to Albert Einstein's second postulate concerning relativity, "The speed of light is independent of the motion of the source." (1905)
Create the reference citation. Using APA formatting for a published paper, the citation would appear as follows: Einstein, A. (1905). On the Electrodynamics of Moving Bodies. In W. Perrett and G.B. Jeffery (Eds.), The Principle of Relativity (891-921). London: Metheun and Company, Ltd.