Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: The Prophet (peace and blessings be upon him) came to Madinah while its people were making salaf sales in dates for two and three years. He (peace and blessings be upon him) said: "Whoever makes a salaf sale in dates, let him do so for a known weight, a known measure and to a known term." In one narration it is stated that the Prophet (peace and blessings be upon him) forbade the sale of a deferred debt for a deferred debt. And it is reported from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that he said: "We see no harm in the salaf sale, when silver is paid for something, the silver being paid in cash."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11109