From Sahih Bukhari (Hadith 3162):
Narrated Juwairiya bin Qudama at-Tamimi:We said to `Umar bin Al-Khattab, "O Chief of the believers! Advise us." He said, "I advise you to fulfill Allah's Covenant (made with the Dhimmis) as it is the convention of your Prophet and the source of the livelihood of your dependents (i.e. the taxes from the Dhimmis)
Reference: Bukhari 3162