Al-Kāfi - Volume 6 > Eating Clay
Hadith #7

أَحْمَدُ بْنُ مُحَمَّدٍ عَنْ مُعَمَّرِ بْنِ خَلَّادٍ عَنْ أَبِي الْحَسَنِ ( عليه السلام ) قَالَ قُلْتُ لَهُ مَا يَرْوِي النَّاسُ فِي أَكْلِ الطِّينِ وَ كَرَاهِيَتِهِ فَقَالَ إِنَّمَا ذَاكَ الْمَبْلُولُ وَ ذَاكَ الْمَدَرُ .

7. Ahmad ibn Muhammad has narrated from Mu’ammar ibn Khallad who has said the following: “I once asked abu al-Hassan, ‘Alayhi al-Salam, about what people narrate as regards eating clay and that it is undesirable. He (the Imam) said, ‘It is clay and chunks of dirt.’”