Muhammad bin Zayd bin Khulaydah said: I asked Sayyiduna Ibn Umar (may Allah be pleased with them both) about the salaf sale. I said to him: We make salaf sales and say: If you give us wheat, we will take it at such-and-such a rate, and if you give us dates, at such-and-such a rate. He replied: Make a salam sale in each kind of commodity for a known amount of silver. If he gives it to you, (well and good); otherwise take back your capital, and do not sell it in exchange for other goods.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11154