Al-Kāfi - Volume 6 > Clipping the finger Nails
Hadith #7

عَنْهُ عَنْ مُحَمَّدِ بْنِ عَلِيٍّ عَنِ الْحَكَمِ بْنِ مِسْكِينٍ عَنْ حُذَيْفَةَ بْنِ مَنْصُورٍ عَنْ أَبِي عَبْدِ اللَّهِ ( عليه السلام ) قَالَ إِنَّ أَسْتَرَ وَ أَخْفَى مَا يُسَلِّطُ الشَّيْطَانَ مِنِ ابْنِ آدَمَ أَنْ صَارَ أَنْ يَسْكُنَ تَحْتَ الْأَظَافِيرِ .

7. It is narrated from the narrator of the previous Hadith from Muhammad ibn Ali from al-Hakam ibn Miskin from Hudhayfah ibn Mansur who has said the following: “Abu ‘Abd Allah, Alayhi al-Salam, has said, ’The strongest cover and most secret place for Satan by which he dominates children of Adam, Alayhi al-Salam is his hiding under the fingernails.’”