Musnad Abu Ya'la al-Mawsili 109:5

2004. Ishaq narrated to us, Malik bin Su'ayr bin al-Khums narrated to us, Isma'il bin 'Abd al-Malik narrated to us, from Abu al-Zubayr, from Jabir, who said: When it was the [Battle of the] Trench, I looked at the Messenger of Allah, may Allah bless him and grant him peace, and I found that he had placed a stone between himself and his lower garment to straighten his back due to hunger.