We made a journey to Syria on an expedition along with the Messenger of Allah (ﷺ). The Nabateans of Syria came to us and we paid in advance to them (in a salam contract) in wheat and olive oil at a specified rate and for a specified time. He asked (by the people): you might have contracted with him who had these things in his possession? He replied: We did not ask them.
Sunan Abi Dawud Book 24, Hadith 51