Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) "forbade selling the fruit on the date palms for dried dates by measure, raisins for grapes by measure, and standing crops for wheat by measure."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10517