Al-Hasan al-Basri (may Allah have mercy on him) said: Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) wrote to Sayyiduna Umar (may Allah be pleased with him) that when Muslim traders go into the Abode of War (dar al-harb), a tenth is collected from them. Sayyiduna Umar (may Allah be pleased with him) replied: When their people come (to us), collect a tenth from them; collect a twentieth from the dhimmi traders; and from the Muslims five dirhams are to be collected on two hundred dirhams, and on whatever exceeds that, one dirham on every forty dirhams.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18770