Language:
كتاب الجهاد والسير
56
Fighting for the Cause of Allah (Jihaad)
(174)
Chapter: One should fight for the protection of the Dhimmi
(174)
باب يُقَاتَلُ عَنْ أَهْلِ الذِّمَّةِ، وَلاَ يُسْتَرَقُّونَ

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."

حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ حُصَيْنٍ، عَنْ عَمْرِو بْنِ مَيْمُونٍ، عَنْ عُمَرَ ـ رضى الله عنه ـ قَالَ وَأُوصِيهِ بِذِمَّةِ اللَّهِ وَذِمَّةِ رَسُولِهِ صلى الله عليه وسلم أَنْ يُوفَى لَهُمْ بِعَهْدِهِمْ، وَأَنْ يُقَاتَلَ مِنْ وَرَائِهِمْ، وَلاَ يُكَلَّفُوا إِلاَّ طَاقَتَهُمْ‏.‏
Reference : Sahih al-Bukhari 3052
In-book reference : Book 56, Hadith 258
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 287
  (deprecated numbering scheme)