Al-Kāfi - Volume 3 > One’s Stepping over Feces or Other Such Things
Hadith #2

2ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنْ حَمَّادٍ عَنْ حَرِيزٍ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ قَالَ كُنْتُ مَعَ أَبِي جَعْفَرٍ (عَلَيْهِ الْسَّلام) إِذْ مَرَّ عَلَى عَذِرَةٍ يَابِسَةٍ فَوَطِىَ عَلَيْهَا فَأَصَابَتْ ثَوْبَهُ فَقُلْتُ جُعِلْتُ فِدَاكَ قَدْ وَطِئْتَ عَلَى عَذِرَةٍ فَأَصَابَتْ ثَوْبَكَ فَقَالَ أَ لَيْسَ هِيَ يَابِسَةً فَقُلْتُ بَلَى فَقَالَ لاَ بَأْسَ إِنَّ الأرْضَ تُطَهِّرُ بَعْضُهَا بَعْضاً.

2. Ali ibn Ibrahim has narrated from his father, from Hammad, from Hariz from Muhammad ibn Muslim who has said the following: “Once I was in the company of abu Ja‘far, ‘Alayhi al-Salam, and on passing (stepping on) some dried up human feces, certain pieces came in contact with his clothes. I informed him about it and he (the Imam) asked, ‘Was it not dried up?’ I said, ‘Yes, it was dried up.’ He (the Imam) said, ‘It (touching) then is not harmful. Certain parts of earth cleanse other parts.’”