From Sahih Muslim (Hadith 3977):
Anas (Allah be pleased with him) reported that Allah's Apostle (ﷺ) forbade the sale of the fruit of date-palms until it becomes mellow. We (some of the other narrators in the chain of transmitters) said:What does the word" mellow" mean? He said: (There the fruit) turns red or yellow. Don't you see if Allah had checked (the growth of) fruits; then what for the wealth of your brother would be permissible for you?
Reference: Muslim 3977