Narrated Ibn Abi Najih:as above, saying, "He should pay the price in advance for a specified measure and for a specified period."
حَدَّثَنَا عَلِيٌّ، حَدَّثَنَا سُفْيَانُ، قَالَ حَدَّثَنِي ابْنُ أَبِي نَجِيحٍ، وَقَالَ، " فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ إِلَى أَجَلٍ مَعْلُومٍ ".
Bukhari
Sales in which a Price is paid for Goods to be Delivered Later (As-Salam) • Hadith 4