Ziyad bin Hudayr (may Allah have mercy on him) said: We did not collect a tenth from a Muslim or from a mu'ahad (one under a covenant). The narrator said: I asked: From whom did you collect the tithe? He said: We collected a tenth from the harbi traders, because when we went to them, they too collected a tenth from us.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18772