Musnad Ahmad 28:427

Nasr ibn Bab narrated to us, from Hajjaj, from Amr ibn Shu'ayb, from his father, from his grandfather, that the Messenger of Allah (may Allah bless him and grant him peace) wrote a document between the Emigrants (Muhajirun) and the Helpers (Ansar), stipulating that they shall pay their blood money (diya) together, and they shall ransom their captives with kindness, and [that there shall be] reconciliation between the Muslims.