Zayd bin Thabit (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) "forbade selling fruits before they ripen," but "granted a concession in 'araya." And 'ariyyah is a date palm set aside for a people; they then obtain dried dates according to the estimate of its fruit.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10660