Musnad Ahmad 26:829

Ya'qub narrated to us, my father narrated to us, from Salih. Ibn Shihab said: Ubayd Allah ibn Abd Allah ibn Utbah narrated to me, that Abd Allah ibn Mas'ud said: While we were with the Messenger of Allah, peace and blessings of Allah be upon him, among nearly eighty men of Quraysh—there was no one among them except a Qurayshi—by Allah, I have never seen the faces of men more beautiful than their faces on that day. They mentioned women and spoke about them, and he spoke with them until I wished that he would be silent. He said: Then he came to me and testified, then he said: "As for what follows, O assembly of Quraysh, you are the people of this matter as long as you do not disobey Allah. But if you disobey Him, He will send upon you those who will strip you as this rod is stripped"—referring to a rod in his hand. Then he stripped his rod, and it became white and smooth.