Sahih Muslim Book 020, Hadith Number 4576.
Chapter : It is good on the part of a leader to take the oath of allegiance from his forces when intending to fight, and an account of the allegiance of God's pleasure under the tree.
It has been narrated on the authority of Jabir who said: We were one thousand and four hundred on the Day of Hudaibiya. We swore fealty to hiin (the Holy Prophet) and 'Umar was holding the latter's hand (when he was sitting) under the tree (called) Samura (to administer the oath to the Companions). The narrator added: We took oath to the effect that we would not flee (from the battlefield if there was an encounter with the Meccans), but we did not take oath to fight to death.