Sahih al-Bukhari, Volume 3, Book 35, Hadith 444 (USC-MSA) ⓘ
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 " .
also referenced under: Book 35, Hadith 4 (In-Book) & Hadith 2240 (DarusSalam)