Al-Kāfi - Volume 6 > The Under arms
Hadith #4

عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ أَحْمَدَ بْنِ أَبِي عَبْدِ اللَّهِ عَنْ مُحَمَّدِ بْنِ عَلِيٍّ عَنْ سَعْدَانَ قَالَ كُنْتُ مَعَ أَبِي بَصِيرٍ فِي الْحَمَّامِ فَرَأَيْتُ أَبَا عَبْدِ اللَّهِ ( عليه السلام ) يَطْلِي إِبْطَهُ فَأَخْبَرْتُ بِذَلِكَ أَبَا بَصِيرٍ فَقَالَ لَهُ جُعِلْتُ فِدَاكَ أَيُّمَا أَفْضَلُ نَتْفُ الْإِبْطِ أَوْ حَلْقُهُ فَقَالَ يَا أَبَا مُحَمَّدٍ إِنَّ نَتْفَ الْإِبْطِ يُوهِي أَوْ يُضْعِفُ احْلِقْهُ .

4. A number of our people have narrated from Ahmad ibn abu as from Muhammad ibn Ali from Sa‘dan who has said the following: “I once was with abu Basir in a bathhouse and I saw abu ‘Abd Allah, ‘Alayhi al-Salam, applying lime on his underarms. I informed abu Basir about it who asked saying, ‘I pray to Allah to keep my soul in service for your cause, which is better; pulling out or shaving the hairs of the underarms?’ He (the Imam) said, ‘O abu Muhammad pulling out the hairs of the underarms weakens (the shoulders) so you must shave them.’”