If you want a specific citation about the impact (or lack of impact) of outliers on Likert scales, could you provide more context regarding what the impact would be on and your sample size? For example, here is a paper on the impact of outliers in Likert scales on alpha. But, the answer will change depending on what kind of analyses you are running. Some analyses are more robust or sensitive to outliers.
As Pavel noted above, it is useful to try and determine the cause of outliers. However, in my experience, you often can have a reasonable guess, but not know the cause. The cause of the outlier and the impact on specific analyses will affect what you do with it. For instance, see here and here.
In my experience, if you don't have strong evidence to support removal (e.g., a lot of deviant responding), I have found it beneficial and reviewer-friendly to run analyses with and without the outlier(s) and report whether the results changed at all. It's more work re-running analyses, but ultimately it is transparent and saves you from trying to argue for either their inclusion or removal (often on grounds that make assumptions).