صحيح البخاري Sahih Bukhari كتاب الجهاد والسير The Book of Jihad (Fighting For Allah’S Cause)
بَابُ يُقَاتَلُ عَنْ أَهْلِ الذِّمَّةِ وَلاَ يُسْتَرَقُّونَ:

(174) Chapter. One should fight for the protection of the Dhimmi (i.e., free non-Muslim subjects living in a Muslim country) and they should not be enslaved.

1 hadith
Hadith 3052
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ ، حَدَّثَنَا أَبُو عَوَانَةَ ، عَنْ حُصَيْنٍ ، عَنْ عَمْرِو بْنِ مَيْمُونٍ ، عَنْ عُمَرَ رَضِيَ اللَّهُ عَنْهُ ، قَالَ : " وَأُوصِيهِ بِذِمَّةِ اللَّهِ وَذِمَّةِ رَسُولِهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنْ يُوفَى لَهُمْ بِعَهْدِهِمْ ، وَأَنْ يُقَاتَلَ مِنْ وَرَائِهِمْ وَلَا يُكَلَّفُوا إِلَّا طَاقَتَهُمْ " .
Narrated `Amr bin Maimun: `Umar (after he was stabbed), instructed (his would-be-successor) saying, "I urge him (i.e. the new Caliph) to take care of those non-Muslims who are under the protection of Allah and His Apostle in that he should observe the convention agreed upon with them, and fight on their behalf (to secure their safety) and he should not over-tax them beyond their capability."
Hadith Reference Sahih Bukhari / The Book of Jihad (Fighting For Allah’S Cause) / Hadith: 3052
Hadith Grading محدثین: أحاديث صحيح البخاريّ كلّها صحيحة