“I once asked abu ’Abd Allah (a.s.), about what is clay on which Sajdah (prostration) cannot be performed. He (the Imam) said, ‘Clay is that on which one’s forehead sinks lower and lower without remaining in one place on earth.’ I asked about a man who performs Salah (prayer) on graves. He (the Imam) said, ‘It is not permissible unless he places something between himself and the graves when performing Salah (prayer), like a distance of ten yards from his front side and from his back side, ten yards from the right and ten yards from the left; then he can perform Salah (prayer) if he wanted.’”