Hadith 8:55
He said that God’s Messenger forbade selling a quantity of dates whose measure was unknown for a specific quantity of dates. Muslim transmitted it.
وَعَنْهُ قَالَ: نَهَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَنْ بَيْعِ الصُّبْرَةِ مِنَ التَّمْرِ لَا يُعْلَمُ مَكِيلَتُهَا بِالْكَيْلِ الْمُسَمَّى مِنَ التَّمْرِ. رَوَاهُ مُسلم
Original Reference: Mishkat al-Masabih 2816