Al-Kāfi - Volume 7 > Testimony of the Members of all other Nations
Hadith #4

4- عَلِيٌّ عَنْ مُحَمَّدِ بْنِ عِيسَى عَنْ يُونُسَ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ عَنْ أَحَدِهِمَا (عَلَيْهِما السَّلام) قَالَ سَأَلْتُهُ عَنِ الصَّبِيِّ وَالْعَبْدِ وَالنَّصْرَانِيِّ يَشْهَدُونَ بِشَهَادَةٍ فَيُسْلِمُ النَّصْرَانِيُّ أَتَجُوزُ شَهَادَتُهُ قَالَ نَعَمْ.

4. Ali has narrated from Muhammad ibn ‘Isa from Yunus from Muhammad ibn Muslim who has narrated the following: “I once asked one of the two Imam, (abu Ja’far or abu ‘Abd Allah), ‘Alayhim al-Salam, about a child, a slave and a Christian who bear testimony; then the Christian becomes a Muslim, if his testimony becomes admissible. He (the Imam) said, ‘Yes, it becomes admissible.’”