The eleventh commandment. Thou shalt not be found out.
“After all, that Eleventh Commandment is the only one that is vitally important to keep in these days.” —B. H. Buxton: Jennie of the Prince's, iii. 314.
The ten commandments. The ten fingers or nails. (Shakespeare: 2 Henry VI., i. 3.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894