Sayyiduna Anas bin Malik (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) forbade selling fruits before they ripen. It was asked: What is its ripening? He (peace and blessings be upon him) said: "When they turn red." He (peace and blessings be upon him) said: "What do you think: when Allah withholds the fruit, then why do you consider your brother's wealth permissible?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10623