Musnad Ahmad 32:1204

Rawh narrated to us, Shu'bah narrated to us, he said: Musa ibn Anas informed me, he said: I heard Anas ibn Malik saying: A man said: "O Messenger of Allah, who is my father?" He said: "Your father is so-and-so." Then it was revealed: {O you who have believed, do not ask about things which, if they are shown to you, will distress you} [Al-Ma'idah: 101], until the completion of the verse.