Sahih Muslim Book 10, Hadith Number 3643.

Chapter : It is invalid to sell the commodity before taking possession of it.

Ibn Abbas (Allah be pleased with them) reported Allah’s Messenger (may peace be upon him) as saying: He who buys food-grain should not sell it, until he has weighed it (and then taken possession of it). I (Tawus) said to Ibn Abbas (Allah be pleased with them): Why is it so? Thereupon he said: Don’t you see that they (the people) sell food-grains against gold for the stipulated time. Abu Kuraib did not make any mention of the stipulated time.