Sahih al-Bukhari 35:1
Narrated Ibn `Abbas:
Allah's Messenger (ﷺ) came to Medina and the people used to pay in advance the price of fruits to be delivered within one or two years. (The sub-narrator is in doubt whether it was one to two years or two to three years.) The Prophet (ﷺ) said, "Whoever pays money in advance for dates (to be delivered later) should pay it for known specified weight and measure (of the dates).
Reference
Sahih al-Bukhari 35:1
In-book reference
Book 35, Hadith 1
Arabic Text
حَدَّثَنَا عَمْرُو بْنُ زُرَارَةَ، أَخْبَرَنَا إِسْمَاعِيلُ ابْنُ عُلَيَّةَ، أَخْبَرَنَا ابْنُ أَبِي نَجِيحٍ، عَنْ عَبْدِ اللَّهِ بْنِ كَثِيرٍ، عَنْ أَبِي الْمِنْهَالِ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ قَدِمَ رَسُولُ اللَّهِ صلى الله عليه وسلم الْمَدِينَةَ، وَالنَّاسُ يُسْلِفُونَ فِي الثَّمَرِ الْعَامَ وَالْعَامَيْنِ ـ أَوْ قَالَ عَامَيْنِ أَوْ ثَلاَثَةً. شَكَّ إِسْمَاعِيلُ ـ فَقَالَ " مَنْ سَلَّفَ فِي تَمْرٍ فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ، وَوَزْنٍ مَعْلُومٍ ".