Kāmil al-Ziyārāt > Eating from any Clay is Unlawful Except the Clay of the Grave of Ĥusain (a.s.), For Verily It Is the Cure
Hadith #3

3 حَدَّثَنِي أَبُو عَبْدِ اللَّهِ مُحَمَّدُ بْنُ أَحْمَدَ بْنِ يَعْقُوبَ عَنْ عَلِيِّ بْنِ الْحَسَنِ بْنِ عَلِيِّ بْنِ فَضَّالٍ عَنْ أَبِيهِ عَنْ بَعْضِ أَصْحَابِنَا عَنْ أَحَدِهِمَا (ع) قَالَ إِنَّ اللَّهَ تَبَارَكَ وَ تَعَالَى خَلَقَ آدَمَ (ع) مِنْ طِينٍ فَحَرَّمَ الطِّينَ عَلَى وُلْدِهِ قَالَ فَقُلْتُ مَا تَقُولُ فِي طِينِ قَبْرِ الْحُسَيْنِ (ص) فَقَالَ يَحْرُمُ عَلَى النَّاسِ أَكْلُ لُحُومِهِمْ وَ يَحُلُّ عَلَيْهِمْ أَكْلُ لُحُومِنَا- وَ لَكِنِ الشَّيْءُ الْيَسِيرُ مِنْهُ مِثْلُ الْحِمَّصَةِ

3. Abū ‘Abdillāh Muĥammad ibn Aĥmad ibn Ya’qūb narrated to me from ‘Ali ibn Ĥasan ibn ‘Ali ibn Faďďāl, from his father, from some of our companions, who said: Abū Ja’far (Imam Bāqir (a.s.)) or Abū ‘Abdillāh (Imam Šādiq (a.s.)) said: Verily Allāh, the Blessed and the Exalted, created Ādam from clay. Therefore, He forbade (eating) clay on the children of Ādam (mankind). I asked, “What about (eating some of) the clay of the grave of Ĥusain (a.s.)?” Imam (a.s.) replied, “It is unlawful for people to eat their own flesh but is it permissible for them to eat our flesh?!!! It is permissible to eat a very little amount from it, as much as a chick-pea.”