Ziyad bin Hudayr (may Allah have mercy on him) said: I wrote to Sayyiduna Umar (may Allah be pleased with him) that harbi disbelievers come into the territory of the Muslims and stay there. Sayyiduna Umar (may Allah be pleased with him) said: If they stay six months, collect a tenth from them; if they stay a whole year, collect a twentieth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18771