Musnad Ahmad 1091:0

Narrated Nu'man ibn Muqarrin: I was with the Messenger of Allah (ﷺ) when he would not fight at the beginning of the day. He would wait until the sun had passed its zenith, the winds had blown, and the help of Allah had descended. He said: "When you meet the enemy, let your shields be of leather. If you are defeated—and I seek refuge in Allah from that—then do not let your shields be of wood, for they will not protect you." He then said: "I heard the Messenger of Allah (ﷺ) say: 'The one who is killed in the path of Allah is a martyr, and the one who dies in the path of Allah is a martyr, and the one who dies of the plague in the path of Allah is a martyr, and the one who dies of a stomach ailment in the path of Allah is a martyr.'"