Musannaf Ibn Abi Shayba 3:815

Waki' narrated to us, he said: Fudayl ibn Ghazwan narrated to us, from Abu Hazim, from Abu Hurayrah, who said: I saw seventy of the people of the Suffah, and among them was one whose garment reached his knees, and among them was one who was below that. So when he would bow, he would grasp it, fearing that his nakedness might be exposed.