Al-Rabi' bin Sulayman says: We said to Imam al-Shafi'i (may Allah have mercy on him): Ali bin Ma'bad (may Allah have mercy on him) narrates with his chain from the Prophet (peace and blessings be upon him) that he permitted selling wheat in its ears when the ears turn white. Imam al-Shafi'i said: "As for that, it is deception, because there is a barrier around it and it cannot be seen. If this report is established from the Prophet (peace and blessings be upon him), we will hold by it, and it will be a specific case taken out of a general rule, because the Messenger of Allah (peace and blessings be upon him) forbade the sale involving deception and permitted this."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10611