Sahih Bukhari Volume 003, Book 035, Hadith Number 447.

Narated By Muhammad bin Al-Majalid : Abdullah bin Shaddad and Abu Burda sent me to 'Abdullah bin Abi Aufa and told me to ask 'Abdullah whether the people in the life-time of the Prophet used to pay in advance for wheat (to be delivered later). Abdullah replied, "We used to pay in advance to the peasants of Sham for wheat, barley and olive oil of a known specified measure to be delivered in a specified period." I asked (him), "Was the price paid (in advance) to those who had the things to be delivered later?" Abdullah bin Aufa replied, "We did not use to ask them about that." Then they sent me to 'Abdur Rahman bin Abza and I asked him. He replied, "The companions of the Prophet used to practice Salam in the life-time of the Prophet; and we did not use to ask them whether they had standing crops or not."