Sahih al-Bukhari 2927
In-book no 2927حَدَّثَنَا 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.
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."