It is narrated from Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight the people until they say: 'There is no god but Allah.' When they say 'There is no god but Allah,' their lives and property become protected from me, except for its due right (i.e., in cases such as retribution, etc.), and their reckoning is with Allah." Abu Bakr (may Allah be pleased with him) said: This is included in its due right. If they withhold even a rope used for tying a camel, which they used to give to the Messenger of Allah (peace and blessings be upon him), I will fight them for it.
Hadith Referenceمسند الإمام الشافعي / كتاب الزكاة / 679
Hadith Takhrijاخرجه البخاري، الجهاد والسير، باب دعاء النبي الى الإسلام والنبوة..... الخ (2946) ومسلم الایمان، باب الأمر بقتال الناس حتى يقولوا.... الخ (21)۔