Deutsch, M., & Gerard, H. B. (1955). A study of normative and informational social influences upon individual judgment. The Journal of Abnormal and Social Psychology, 51(3), 629-636.
An overview on PsychWiki.
the original article’s abstract:
The effects of two different types of social influence upon individual judgment were investigated: normative and informational. Prior studies of ‘group’ influence were shown to involve only incidentally the type of influence most specifically associated with groups, normative influence. The role of normative influence in buttressing as well as undermining individual experience was also investigated.
And some background by Morton Deutsch, 25 years later.