“…And bravery is defined by two things:
a) the strength and firmness of the heart in the face of fears, and
b) physical strength when fighting, such that one can kill with immense force and magnitude.
The first is the definition of bravery. As for the latter, it indicates physical strength and ability, and not everyone who possesses physical strength has strength of heart, and vice versa.
Because of this, you may find that it is said regarding a man who kills many people: ‘He would do this if he had with him those who could guarantee his safety.’ But, if he becomes scared, he is stricken with cowardice, and his heart becomes detached. And you would find the man with a firm heart who has not killed many people with his own hands firm in the face of fears, going forth in the face of hardships, and this is a characteristic that is required by the commanders, leaders, and forerunners of war, more so than the other, as the forerunner, if he is brave and firm at heart, will go forth and remain firm and will not be defeated, and his supporters will fight alongside him. If he was a coward and weak at heart, he will be humiliated, will not go forth, and will not remain firm, even if he is physically strong.
And the Prophet was the most complete in regards to this bravery that is appropriate for the commanders in war, and he did not kill with his hand anyone except Ubayy bin Khalaf. He killed him on the day of Uhud, and did not kill anyone else with his hand before or after this.”
[‘Minhaj as-Sunnah an-Nabawiyyah’; 8/78]