There have been several classic experiments to study and describe classical conditioning; one of the more famous is the Little Albert experiment. Over the years, the experiment has lost some of its validity due to numerous interpretations by several introductory psychology textbooks. It is, therefore, beneficial to go back to the original (or primary) source and read what the authors of the experiment themselves had to say.
The original article by Watson and Rayner was published in the Journal of Experimental Psychology in 1920. There is a reprint of the original article which can be found in the South University Online Library. Here is the reference for the article reprint: Watson, J. B.,