Musannaf Ibn Abi Shayba 16:586

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Mansur, from Abu Wa’il, from Ibn Mas’ud, that he used to let his lower garment hang low (isbal). It was said to him regarding that, so he said: "Verily, I am a man with thin shins."