Sahih Muslim Book 19, Hadith Number 4411.

It has been reported on the authority of Hudbaifa b. al-Yaman who said: Nothing prevented me from being present at !he Battle of Badr except this incident. I came out with my father Husail (to participate in the Battle), but we were caught by the disbelievers of Quraish. They said: (Do) you intend to go to Muhammad? We said: We do not intend to go to him, but we wish to go (back) to Medina. So they took from us a covenant in the name of God that we would turn back to Medina and would not fight on the side of Muhammad (may peace be upon him). So, we came to the Messenger of Allah (may peace be upon him) and related the incident to him. He said: Both, of you proceed (to Medina) ; we will fulfil the covenant made with them and seek God’s help against them.