Ibrahim al-Nakha'i (may Allah have mercy on him) said: A man came to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) and said: I have accepted Islam, so remove the tax. He (may Allah be pleased with him) said: No, your land was taken by fighting. The narrator said: Another man came (and said) that the people of such-and-such land are able to pay more kharaj. He (may Allah be pleased with him) said: We made peace with them on this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18416