Sufyan bin Abd al-Malik (may Allah have mercy on him) said: It was mentioned before Abdullah bin al-Mubarak (may Allah have mercy on him) that eating clay is unlawful, and he rejected it and said: If this were the saying of the Prophet (peace and blessings be upon him), I would have placed it on my head and eyes and obeyed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19720