It is narrated from Anas bin Malik (may Allah be pleased with him) that the Prophet (ﷺ) forbade the sale of dates on the date palm before they reach the stage of "zahu." He was asked, "What is zahu?" He replied: "It is when they become red."
Hadith Referenceمسند الإمام الشافعي / كتاب البيوع / 1420