you will not find such a generic assertion because it is so obviously wrong that nobody would dare to assert it.Or because nobody actually believes it. Even the more staunch laissez-faire defender believes some self-beneficial acts to be contrary to the common welfare, like stealing or getting reelected after promoting anti-market policies.
Likewise, anti-capitalists argue that many self-beneficial actions are also beneficial to society, like demanding to be paid fair wage or even taking control of their means of production. "Workers, unite!" is not a call for self-sacrifice.
People cherry-pick the bits and pieces that make their actions look good and ignore the rest.
Sure, but that's not an economic theory.