Imam al-Shafi'i (may Allah have mercy on him) said: "Because by the saying of the Prophet (peace and blessings be upon him), 'Whoever makes a salaf (salam sale), let him make the salaf,' he (peace and blessings be upon him) meant: 'Let him pay (the price).' The term taslif (advance payment) does not apply to it until he pays him what he has to pay in advance before he parts from the one with whom he made the salaf contract."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / Q11109
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