Allah had warned Prophet Muhammad in the Quran (64:4) that his companions contained hypocrites so skilled at deceit that the prophet himself was pleased by their speech and appearance. Only revelation could expose them.
Later, during the return from the Tabuk expedition, while Prophet Muhammad went through a mountain pass while instructing his army to pass from below, some of these hypocrites tried to assault the prophet by startling his horse into dropping him off the cliff. Their plan was foiled and they fled. Despite wearing masks to hide their identies, they could be known by their horses.
The Quran (9:47) said that these hypocrites had renounced faith after becoming Muslims out of greed and vindictiveness.
However, instead of punishing those men, Prophet Muhammad decided to not pursue them because of the division it could cause amongh the Muslims. Those were men who appeared to be highly respected (as Quran 64:4 implied) and accusations against them would risk causing fitna.
Only Hudhayfah ibn Al-Yaman was told their names, and Prophet Muhammad instructed him to keep it a secret.
Lecture notes available at http://www.why-quran.org/?p=731
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