The Prophet ﷺ forbade the sale of grapes till they became black and the sale of grain till it had become hard. (Using translation from Abū Dāʾūd 3371)
أَنَّ رَسُولَ اللهِ ﷺ نَهَى عَنْ بَيْعِ الثَّمَرَةِ حَتَّى تَزْهُوَ وَعَنْ بَيْعِ الْحَبِّ حَتَّى يَشْتَدَّ وَعَنْ بَيْعِ الْعِنَبِ حَتَّى يَسْوَدَّ