Sulayman bin Buraydah narrates from his father (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said about the dhimmis (non-Muslim subjects): "For them it is that if they accept Islam, then on their wealth, slaves, homes, land and livestock there is only zakah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7500