From Sahih Bukhari (Hadith 3052):
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: Bukhari 3052