Musnad Abu Ya'la al-Mawsili 27:1

Al-Hasan ibn al-Sabbah narrated to us, al-Abbas ibn al-Fadl al-Ansari narrated to us, from Ibn Sinan—meaning Burdah, if Allah wills—from Uqbah ibn Ali, from Yahya ibn Zayd, from Abu Anisah, from Abu Layla, who said: The Messenger of Allah (ﷺ) went out, and we went out with him. He passed by a man from Banu Adi who had his thigh uncovered. The Messenger of Allah (ﷺ) said: "Cover your thigh, O Ma'mar, for the thigh is part of the 'awrah (intimate parts)."