It is ironically tempting to cite an appropriate response, such as the words of Sydney Smith (1771-1845), "What you don't know would make a great book." However, since this particular employee seems to be most comfortable in the world of citations and quotations, this may not be the way to go.
There is no particular reason why some people feel compelled to communicate in such a way that their comments need footnotes. For some, there may be underlying insecurities, and flashing this knowledge may make them appear and feel more intellectual. Others are in fact highly intellectual, and this is part of the way that their minds work.
Since you are dealing with a new employee, this may merely be part of the way that he copes with new situations. As he becomes more comfortable, there may be fewer citations.
Regardless, rather than being incited by his tendency to cite, it makes more sense for you to focus on his performance, effectiveness, and contributions to your department. It is possible that his arguably eccentric behavior may lead to increased departmental creativity. At the same time, it may be helpful to take a look at yourself and the possibility that this new employee's expertise may be making you feel a little insecure.
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