Hazrat Ya’laa bin Murrah (radhiyallahu ‘anhu) mentions:

On one occasion during the khilaafah of Hazrat ‘Ali (radhiyallahu ‘anhu), the companions of Hazrat ‘Ali (radhiyallahu ‘anhu) said to one another, “Hazrat ‘Ali (radhiyallahu ‘anhu) is engaged in war and the enemy are nearby. We fear that a surprise attack may be launched against him at any time. Thus, ten people from among us should guard him every night.”