Sayyiduna Ibn Umar (may Allah be pleased with them both) says: A man made a salaf sale in dates with another man, and that year the palms bore no fruit. They brought the dispute to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Why do you consider his wealth lawful for yourself? Return it to him." Then he (peace and blessings be upon him) said: "Do not make salaf sales in dates before they ripen."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11113