Al-Kāfi - Volume 7 > Testimony of the Members of all other Nations
Hadith #3

3- عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ النَّوْفَلِيِّ عَنِ السَّكُونِيِّ عَنْ أَبِي عَبْدِ اللهِ (عَلَيْهِ السَّلام) قَالَ قَالَ أَمِيرُ الْمُؤْمِنِينَ (عَلَيْهِ السَّلام) الْيَهُودُ وَالنَّصَارَى إِذَا شَهِدُوا ثُمَّ أَسْلَمُوا جَازَتْ شَهَادَتُهُمْ.

3. Ali ibn Ibrahim has narrated from his father from al-Nawfaliy from al-Sakuniy who has narrated the following: “Abu ’Abd Allah, ‘Alayhi al-Salam, has said that ’Amir al-Mu’minin, Ali, ‘Alayhi al-Salam, has said, ‘If Jews and Christians bear testimony then become Muslims, their testimony is admissible.’”