Dabbah bin Mihsan (may Allah have mercy on him) said: I said to the Commander of the Believers, Umar bin al-Khattab (may Allah be pleased with him), that Abu Musa (may Allah be pleased with him) had chosen forty of the sons of the Asawirah (Persian cavalry nobles) for himself. When Abu Musa (may Allah be pleased with him) came, Umar (may Allah be pleased with him) said: What is the matter with you, that you have chosen forty sons (of the Asawirah)? He said: O Commander of the Believers, I chose them because I feared that the army would be cheated of them; so I ransomed them and strove hard over their ransom, then I took out the khumus and distributed (the rest). Dabbah (may Allah have mercy on him) said: It is true, by Allah; neither did the Commander of the Believers lie, nor did I lie to him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12851