Musnad Ahmad 28:117

Husayn (meaning Ibn Muhammad) narrated to us, Jarir (meaning Ibn Hazim) narrated to us, from Muhammad (meaning Ibn Ishaq), from Abu Sufyan, from Muslim ibn Jubayr, from Amr ibn al-Hurayth, who said: I asked Abdullah ibn Amr ibn al-As and said: "We are in a land where there is neither dinar nor dirham, and we only trade with camels and sheep on credit. What is your opinion on that?" He said: "You have come to one who is informed. The Messenger of Allah (peace and blessings of Allah be upon him) prepared an army using camels from the camels of charity (Sadaqah) until they ran out, and there were people remaining. The Messenger of Allah (peace and blessings of Allah be upon him) said: 'Purchase camels for us in exchange for young female camels from the camels of charity when they arrive, so that we may pay them back.' So, I purchased one camel in exchange for two or three young female camels, until I finished, and the Messenger of Allah (peace and blessings of Allah be upon him) paid that back from the camels of charity."