I have taught six sigma black belt classes using consultant sourced training materials which included the rule of thumb that when estimating the rate of occurrence of discreet events, like the proportion of defective units produced by a manufacturing process, a sample size large enough to include at least 5 defective units is desired.

For example if the defect rate is 1 in a million, you must sample at least 5,000,000 units to obtain a reasonably accurate estimate of the defect rate.

I need an authoritative reference supporting this rule of thumb