Musannaf Ibn Abi Shayba 16:585

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from 'Amr ibn Murrah, from Ibrahim, from Ibn Mas'ud, who said: A young man entered upon 'Umar, and the young man began to praise him. 'Umar saw him dragging his lower garment, so he said to him: "O son of my brother, raise your lower garment, for it is more righteous for your Lord and cleaner for your garment." He (Ibn Mas'ud) said: 'Abdullah used to say: "How amazing is 'Umar! If he saw the right of Allah upon him, that which he was in (his position of authority) did not prevent him from speaking about it."