It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) forbade selling anything forward before taking possession of it. The narrator asked about its meaning, so he replied that it means that dirhams are exchanged for dirhams and the grain is deferred.
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 2275