Murrah said that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: "There are three things which, had the Messenger of Allah (peace and blessings be upon him) explained them clearly, would have been dearer to me than red camels: the caliphate, kalalah, and usury (riba)." I said to Murrah: Who doubts about kalalah, that it is other than the child and the father? Murrah replied: They doubt about the father.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12279