Sahih Bukhari Volume 3, Book 35, Hadith Number 446.

Narated By Shu’ba : Muhammad or ‘Abdullah bin Abu Al-Mujalid said, “Abdullah bin Shaddad and Abu Burda differed regarding As-Salam, so they sent me to Ibn Abi Aufa and I asked him about it. He replied, ‘In the life-time of Allah’s Apostle, Abu Bakr and ‘Umar, we used to pay in advance the prices of wheat, barley, dried grapes and dates to be delivered later. I also asked Ibn Abza and he, too, replied as above.'”