From Sahih Muslim (Hadith 4807):
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
Reference: Muslim 4807