Sayyiduna Anas bin Malik (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) forbade "selling grain before it hardens, grapes before they turn black, and fruits before they ripen."
Note: The words "the grain hardens and the grapes turn black" are narrated only by Hammad bin Salamah from his teacher Humayd, while the others narrate from Humayd, from Anas, without this mention.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10613