It is narrated from Sayyiduna Yazid bin Amir al-Suwa'i (may Allah be pleased with him) (he had been present at Hunayn on the side of the polytheists, then later embraced Islam); the narrator said: We asked him about the terror that Allah cast into their hearts on the day of Hunayn: what was it like? He picked up a pebble and threw it into a basin, and it rang; then he said: "We used to feel exactly the same within our breasts."
Hadith Referenceالمعجم الكبير للطبراني / باب الياء / 18843