the prophet peace be upon him was the most courageous of people. this is evidenced by the fact that he stood against the unbelievers by himself, calling for tawheed and sincere worship of almighty allah alone. all the unbelievers opposed him and were united in waging war against him. they abused him severely and plotted to kill him several times but that did not terrify or dissuade him; rather, it increased his persistence in da`wah and adherence to the truth that had come to him. challenging the tyrants of the earthhe declared defiantly,
"by allah, if they should place the sun in my right hand and the moon in my left for me to abandon this matter, i will not abandon it until allah makes it dominant or else i die in the effort."
anas ibn malikmay allah be pleased with him said, "the messenger of allah peace be upon him was the best of people, the most generous of people and the most courageous of people. the inhabitants of madinah were frightened by a sound one night so some of them went out toward it, but they met the messenger of allah peace be upon him returning. he peace be upon him had gone out before them to investigate the sound, riding an unsaddled horse belonging to abu talhahmay allah be pleased with him with a sword tied to its neck. he peace be upon him told them, "fear not, fear not." an-nawawi commented, "one recognizes his courage by the extreme haste with which he went out to meet a possible enemy before everyone else; he had investigated the situation and was on his way back before they arrived there.
jabirmay allah be pleased with him reported: "we were digging the trench [to defend the city] when we came upon a hard boulder. they went to the prophet peace be upon him and said, 'this boulder is obstructing [excavation of] the trench.' he peace be upon him said, 'i'm coming down.' hepeace be upon him got up and he peace be upon him had a stone tied against his stomach [from hunger]; we had been three days without tasting food. the prophet peace be upon him took the pickaxe and struck the boulder. it became sand, pouring down. (al-bukhari) this hard rock, which none of the companions could break, turned into a mound of sand from the strength of the prophet's peace be upon him blow; and this is evidence of his strength peace be upon him.
he peace be upon him possessed courage, assertiveness and perseverance in face of the most terrifying circumstances to such a degree that no other has come close to it. this is what enabled the prophet peace be upon him to attend so many battles during his life that was full of struggle. there is not a single report that he ever thought of remaining behind, even the slightest distance. and this is what filled his companions with admiration and love; he was a leader who was obeyed by young and old, even when the order was no more than a signal. it was not only because he was the messenger of allahpeace be upon him, but because of what they observed of his courage, which made their own seem insignificant, although among them were heroes whose bravery was legendary.
`ali ibn abi talibmay allah be pleased with him said, "whenever battle became fierce and the two factions met we would shield ourselves behind the messenger of allah peace be upon him, and none of us would be nearer to the enemy than he peace be upon him." (ahmad and an-nasa'i) and he may allah be pleased with him said, "the day of badr i saw that we sought safety with the prophet peace be upon him. he peace be upon him was the closest of us to the enemy and the fiercest of us in battle." (ahmad)
during the battle of uhud, the accursed ubayy ibn khalaf approached on his horse to kill the prophet peace be upon him and shouted, "o muhammad! there is no escape for me if you should escape!" the muslims said, "o messenger of allah peace be upon him, should a man from among us attack him?" he peace be upon him replied, "leave him." when he came near, the prophet peace be upon him took a spear from al-harith ibn as-sammahmay allah be pleased with him and shook it so violently that the companions around him dispersed. then he peace be upon him faced him and struck him in the neck so that he tumbled from his horse. he returned to the quraysh, saying, "muhammad has killed me." they said, "but you are unharmed." he said, "if it had struck all the people it would have killed them. did he not say, 'i will kill you'? by allah, if he had spat on me it would have killed me." he died on his way back to makkah.
and in the battle of hunayn, the muslims fled when hawazin surprised them with a barrage of arrows. but the prophet peace be upon him stood firm before the enemy and said:
"i am the prophet; it is no lie. i am the son of `abd al-muttalib." 
o allah, send blessings and peace upon your beloved prophet muhammad peace be upon him. let us join him in paradise, your home of honour, and drink from his noble hand, a drink after which we will never feel thirst.
from muhammad, the perfect man, pp.188-189.
as-seerah an-nabawiyyah by ibn hisham, 3/174.
akhlaaq an-nabiyyi fil-qur'an was-sunnah, 3/1341.