In the Battle of Ohud, the Prophet (s.a.w.) ordered some archers to picket the rear defile of the mountain.  "Do not move from here in any circumstances for the enemy might strike from behind these narrow passages."  However, when the archers saw that the enemy was retreating, they thought the battle was over.  They deserted their places and busied themselves collecting the spoils of war.  This eagerness turned the tide against the Muslims.

        Khalid bin Walid, the commander of the infidel cavalry, effected a charge and killed the small guard of 10 men.  He then furiously advanced from the rear, killing Mosaib bin Omair, and causing a great confusion among Muslims.  A voice rang on the mountain of Ohud declaring that the Prophet (s.a.w.) was dead, and this further demoralized them.  Many companions like Omar bin Khattab, Osman bin Affan and Abu Obeidah took flight.

        Anas bin Nadhr having seen those deserters said: "What makes you run away?" They said: "The Prophet is no more.  He has been killed."  Anas said: "What is the value of your lives if the Prophet (s.a.w.) has died.  Let us fight for his cause".  Anas fought alone and was martyred.

        The Prophet (s.a.w.) had been wounded.  The Battle ended in confusion, with infidels having returned to Mecca in belief that the Muslims were routed.  At that time, he inquired about Sa'd bin Rabi.  "Is he alive or dead?  Go find him"; he ordered the handful of faithfuls who were by his side.

        Sa'd bin Rabi had suffered multiple wounds, and had fallen.  When they found him, he was breathing his last.  His last words were "Peace be upon Muhammad for he fulfilled his duty.  If he is alive, give him my Salaam.  And if he is dead, Allah is ever Alive.  Continue to fight for His cause".
 

Reference:
Pearls of Wisdom.  A String of Incidents in the History of Islam.  The World Federation of Khoja Shia Ithna-Asheri Muslim Communities. Page 29.