Anas bin Sirin (may Allah have mercy on him) narrates from his father: Sayyiduna Anas bin Malik (may Allah be pleased with him) made a kitabah contract with me for 20 thousand dirhams. I was among those who conquered Tustar; I bought goods and made a profit. I came to Sayyiduna Anas bin Malik (may Allah be pleased with him) with the kitabah amount, but he refused to accept it, (accepting) only the fixed instalments. I mentioned this to Sayyiduna Umar (may Allah be pleased with him), and he said: "Anas wants the inheritance." And he wrote to him: "Accept it from him." So he accepted it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21707