Al-Kāfi - Volume 6 > Clipping the finger Nails
Hadith #3

مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنِ الْحَسَنِ بْنِ عَلِيٍّ عَنِ الْحَسَنِ بْنِ سُلَيْمَانَ عَنْ عَمِّهِ عَبْدِ اللَّهِ بْنِ هِلَالٍ قَالَ قَالَ لِي أَبُو عَبْدِ اللَّهِ ( عليه السلام ) خُذْ مِنْ شَارِبِكَ وَ أَظْفَارِكَ فِي كُلِّ جُمُعَةٍ فَإِنْ لَمْ يَكُنْ فِيهَا شَيْ‏ءٌ فَحُكَّهَا لَا يُصِيبُكَ جُنُونٌ وَ لَا جُذَامٌ وَ لَا بَرَصٌ .

3. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from al-Hassan ibn Ali ibn Sulayman from his uncle ‘Abd Allah ibn Hilal who has said the following: “Abu ‘Abd Allah, Alayhi al-Salam, once said to me, ’You must clip your mustache and fingernails every Friday, if you do not need clipping then just scratch them. You will not be affected by insanity, leprosy and albinism.’”