From entry five users (the myth) in glossary Statistics for HCI: Making Sense of Quantitative Data
Often it is said that five users are enough for usability testing. This had some truth in the particular original context of iterative software development, where only enough issues needed to be uncovered for the next cycle of re-development. However, it is certainly not sufficient for statistical purposes.
Used in Chap. 4: page 50
Links:
Alan's Blog: Are five users enough?