Musannaf Ibn Abi Shayba 28:147

Hushaym narrated to us, from Yunus, from al-Hasan, from Abu Barzah, who said: We were on an expedition of ours, and we encountered some people from the polytheists. We drove them away from a basket of theirs, and we fell upon it and began to eat from it. We used to hear during the Jahiliyyah that whoever eats bread becomes fat. He said: So when we ate the bread, one of us began to look at his sides to see if he had become fat.