Sayyiduna Anas (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: "It is not permissible to sell dates before they ripen." The Companions asked: "What is the sign of ripening?" He said: "That they become red or yellow."
Hadith Referenceالمنتقى ابن الجارود / كتاب البيوع والتجارات / 604