Sayyiduna Zayd bin Thabit (may Allah be pleased with him) said: Umar (may Allah be pleased with him) came to Abu Bakr (may Allah be pleased with him) and said: Indeed, when the Companions of the Messenger of Allah (peace and blessings be upon him) were martyred on the day of Yamamah, they were falling just as moths fall (then the narrator related the full hadith).
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4825