Musannaf Ibn Abi Shayba 13:402

Hushaym narrated to us, from Isma'il, who said: I said to al-Sha'bi, "Do you see the man who buys something from a man; is it binding upon him to have it witnessed, is it inevitable?" He said, "Do you not see His saying: 'Then if one of you entrusts the other'?"