Abdullah bin Wahb (may Allah have mercy on him) said that Imam Malik (may Allah have mercy on him) was asked about selling the clay which people eat, and he said: I do not like that something which harms people in their religion and their worldly life should be sold. Allah says: "They ask you what has been made lawful for them. Say: Good things have been made lawful for you." [Surah al-Ma'idah: 4]
Imam Malik (may Allah have mercy on him) said: I hold that the market supervisor should prevent them from selling it and forbid it. And Imam Malik (may Allah have mercy on him) said: It is also a kind of foolishness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19721