Narrated Ibn 'Abbas (RA):

When the Prophet (ﷺ) came to al-Madinah, they were paying one and two years in advance for fruits. So he (ﷺ) said: "Those who paid in advance for fruits must do so for a specified measurement and weight, and for a specified time." [Agreed upon].

عَنِ اِبْنِ عَبَّاسٍ ‏-رَضِيَ اَللَّهُ عَنْهُمَا‏- قَالَ: قَدِمَ اَلنَّبِيُّ ‏- صلى الله عليه وسلم ‏-اَلْمَدِينَةَ, وَهُمْ يُسْلِفُونَ فِي اَلثِّمَارِ اَلسَّنَةَ وَالسَّنَتَيْنِ, فَقَالَ: { مَنْ أَسْلَفَ فِي تَمْرٍ فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ, وَوَزْنٍ مَعْلُومٍ, إِلَى أَجَلٍ مَعْلُومٍ } مُتَّفَقٌ عَلَيْهِ [1]‏ .‏