Al-Nakha'i (may Allah have mercy on him) said: Our elders related that on the day of al-Qadisiyyah a man from among the sons of kings fell to the share of (the tribe of) al-Nakha'. Sa'd (may Allah be pleased with him) wanted to take him from them, but they set upon him with their whips. He sent word to them: I have written to Umar bin al-Khattab (may Allah be pleased with him). They said: We are content. So Umar (may Allah be pleased with him) wrote to him: We do not take the khumus from the sons of kings. So Sa'd (may Allah be pleased with him) took him from them. Al-Mughirah (may Allah have mercy on him) said: (That was) because his ransom was more than that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12852