Abu al-Bakhtari says: I asked Abdullah bin Umar (may Allah be pleased with them both) about the salam sale made in dates, and he said: "The sale of dates has been forbidden until they ripen." He says: Then I asked Abdullah bin Abbas (may Allah be pleased with them both), and he said: "The sale of dates has been forbidden until they can be eaten from and can be weighed."
Imam Shu'bah (may Allah have mercy on him) says: I asked a man sitting beside (him): "What is meant by being weighed?" He said: "That it has been picked and secured."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11110