Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: Sayyiduna Usamah bin Zaid (may Allah be pleased with him) informed me of this, that the Prophet Muhammad (peace and blessings be upon him) said: "Trade is done on credit." Abu Bakr Humaydi (may Allah have mercy on him) says: Sufyan sometimes does not narrate this hadith as a 'marfu'' (attributed directly to the Prophet). When he was asked about this, he replied: Sometimes I refrain from narrating this report due to my dislike of 'bay' sarf' (exchange of currency), but as for the 'marfu'' narration, it is indeed 'marfu'.