(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."
Reference: Sahih al-Bukhari 3052
In-book reference: Book 56, Hadith 258
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 4, Book 52, Hadith 287