Al-Kāfi - Volume 5 > Selling Arms to al- Sultan
Hadith #3

3- أَحْمَدُ بْنُ مُحَمَّدٍ عَنْ عَلِيِّ بْنِ الْحَكَمِ عَنْ هِشَامِ بْنِ سَالِمٍ عَنْ مُحَمَّدِ بْنِ قَيْسٍ قَالَ سَأَلْتُ أَبَا عَبْدِ اللهِ (عَلَيْهِ السَّلام) عَنِ الْفِئَتَيْنِ تَلْتَقِيَانِ مِنْ أَهْلِ الْبَاطِلِ أَنَبِيعُهُمَا السِّلاحَ قَالَ بِعْهُمَا مَا يَكُنُّهُمَا كَالدِّرْعِ وَالْخُفَّيْنِ وَنَحْوِ هَذَا.

3. Ahmad ibn Muhammad has narrated from Ali ibn al-Hakam from Hisham ibn Salim from Muhammad ibn Qays who has said the following: “This is concerning my question before abu ‘Abd Allah, ‘Alayhi al-Salam, about the two groups of falsehood who fight each other; if I can sell arms to them. He (the Imam) said, ‘You can sell to them protective materials, like shields and shoes and so on.’”