Musannaf Ibn Abi Shayba 16:645

Abu Bakr narrated to us, he said: Jarir ibn Hazim narrated to us, from Wasil, the freed slave of Ibn Uyaynah, who said: "Indeed, Allah revealed to Ibrahim: 'You are the most noble of creation to Me. So when you pray, do not let the earth see your nakedness, and take for yourself trousers.'"