Messenger of Allah 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).  

قَدِمَ رَسُولُ اللَّهِ ﷺ الْمَدِينَةَ وَالنَّاسُ يُسْلِفُونَ فِي الثَّمَرِ الْعَامَ وَالْعَامَيْنِ أَوْ قَالَ عَامَيْنِ أَوْ ثَلاَثَةً شَكَّ إِسْمَاعِيلُ فَقَالَ مَنْ سَلَّفَ فِي تَمْرٍ فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ  

as above, mentioning only specific measure.  

عَنِ ابْنِ أَبِي نَجِيحٍ بِهَذَا فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ