Mukhtasar Bukhari

Narrated Ibn Abbas (may Allah be pleased with them) that the Messenger of Allah (peace and blessings of Allah be upon him) came to Madinah and at that time people used to do Salam (forward sale) in dates for one or two years. So the Prophet (peace and blessings of Allah be upon him) said: "Whoever does Salam in dates should do so according to a specified measure and a specified weight." In another narration from Ibn Abbas (may Allah be pleased with them), it is reported that the Messenger of Allah (peace and blessings of Allah be upon him) said: "He should do (Salam) in a specified measure for a specified period." (If cash is paid in advance for a certain period and the commodity (crop or any item) is received after that period, this is called "Salam".)