Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade the sale of fruits until they are ripe. They said: What is meant by ripe? He said: When they become red. He added: What if Allah prevents the fruits, how could one of you consider his brother's money lawful for himself?

عن أنس بن مالك رضي الله عنه مرفوعاً: «أن رسول الله صلى الله عليه وسلم نهى عن بيع الثمار حتى تُزْهِي. قيل: وما تُزْهِي؟ قال: حتى تَحمَرَّ. قال: أرأيت إن مَنَعَ اللهُ الثمرة، بِمَ يستحِلُّ أحدُكُم مال أخيه؟».
[صحيح] - [متفق عليه]

Anas ibn Mālik (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) forbade the sale of fruits until they are ripe. They said: "What is meant by ripe?" He said: "When they become red." He added: "What if Allah prevents the fruits, how could one of you consider his brother's money lawful for himself?"
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Fruits are prone to many plant diseases before they become ripe, and selling them in this stage brings about no benefit for the buyer. Therefore, the Prophet (may Allah's peace and blessings be upon him) prohibited the seller and the buyer from concluding the sale of fruits before they become ripe. Dates become ripe when they turn red or yellow. Then, the Prophet (may Allah's peace and blessings be upon him) explained the reason behind prohibiting the sale of unripe fruits by mentioning that if all the fruits or some of them suffered blight, then how could the seller consider his brother's money lawful for himself without giving anything in exchange?