Sunan an-Nasa'i 4616
It was narrated that Abu Al-Minhal said:
"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'''
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ ابْنِ أَبِي نَجِيحٍ، عَنْ عَبْدِ اللَّهِ بْنِ كَثِيرٍ، عَنْ أَبِي الْمِنْهَالِ، قَالَ سَمِعْتُ ابْنَ عَبَّاسٍ، قَالَ قَدِمَ رَسُولُ اللَّهِ صلى الله عليه وسلم الْمَدِينَةَ وَهُمْ يُسْلِفُونَ فِي التَّمْرِ السَّنَتَيْنِ وَالثَّلاَثَ فَنَهَاهُمْ وَقَالَ ‏
"‏ مَنْ أَسْلَفَ سَلَفًا فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ إِلَى أَجَلٍ مَعْلُومٍ ‏"‏ ‏.‏