It is narrated from Ibn Abbas (may Allah be pleased with them) that he said: What the Messenger of Allah (peace be upon him) forbade is the sale of grain which is not taken into possession. Ibn Abbas (may Allah be pleased with them) said from his own opinion, I consider this rule to apply to everything.
Hadith Referenceمسند الإمام الشافعي / كتاب البيوع / 1403