Musannaf Ibn Abi Shayba 26:440

Abu Usama narrated to us, from Hisham ibn Urwah, from his father, from Yahya ibn Abd al-Rahman ibn Hatib, from his father, that Umar judged regarding two men who claimed a man [as their father], and it was not known which of them was his father. Umar said to the man, "Follow whichever of them you wish."