The Prophet ﷺ came (to Medina) and he told the people (regarding the payment of money in advance that they should pay it) for a known specified measure and a known specified weight and a known specified period.
سَمِعْتُ ابْنَ عَبَّاسٍ ؓ يَقُولُ قَدِمَ النَّبِيُّ ﷺ وَقَالَ فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ إِلَى أَجَلٍ مَعْلُومٍ
"I heard Ibn 'Abbas say: 'When the Prophet came (to al-Madinah), they used to pay in advance for dates, two or three years in advance. He forbade them that and said: 'whoever pays in advance for dates, let him pay for a known amount or a known weight, to be delivered at a known time''' (Using translation from Nasāʾī 4616)
قَالًًًَ سَمِعْتُ ابْنَ عَبَّاسٍ يَقُولُ قَدِمَ رَسُولُ اللهِ ﷺ الْمَدِينَةَََََ وَهُمْ يُسْلِفُونَ فِي التَّمْرِ السَّنَتَيْنِ وَالثَّلَاثَ فَنَهَاهُمْ وَقَالَ «مَنْ أَسْلَفًَ سَلَفًا فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ إِلَى أَجْلٍ مَعْلُومٍ»
"I heard Ibn 'Abbas say: 'When the Prophet came (to al-Madinah), they used to pay in advance for dates, two or three years in advance. He forbade them that and said: 'whoever pays in advance for dates, let him pay for a known amount or a known weight, to be delivered at a known time''' (Using translation from Nasāʾī 4616)
سَمِعْتُ ابْنَ عَبَّاسٍ يَقُولُ قَدِمَ رَسُولُ اللهِ ﷺ الْمَدِينَةَ وَهُمْ يُسْلِفُونَ فِي التَّمْرِ السَّنَتَيْنِ وَالثَّلَاثَ فَنَهَاهُمْ وَقَالَ «مَنْ أَسْلَفَ سَلَفًا فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ إِلَى أَجَلٍ مَعْلُومٍ»
"مَنْ أَسْلَفَ في شَيْءٍ، فَليُسْلِفْ في كَيلٍ مَعْلومٍ وَوزْنِ مَعلُومٍ إِلى أَجَلٍ مَعْلُومٍ".