Harb bin Ubaydullah narrates from his father, and he from Abu Hamidah (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "There are no tithes upon the Muslims; the tithes are only upon the Jews and Christians."
(b) Ata' bin al-Harith al-Thaqafi narrates that his father informed him, and he was among those who came in a delegation to the Prophet (peace and blessings be upon him).
If this is authentic, then what is meant, and Allah knows best, is the tithing of their wealth when they come and go for trade; once they accept Islam, that is lifted from them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18707