Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 405
Narrated Qatada:
We do not know of any tribe amongst the 'Arab tribes who lost more martyrs than Al-Ansar, and they will have superiority on the Day of Resurrection. Anas bin Malik told us that seventy from the Ansar were martyred on the day of Uhud, and seventy on the day (of the battle of) Bir Ma'una, and seventy on the day of Al-Yamama. Anas added, "The battle of Bir Ma'una took place during the lifetime of Allah's Apostle and the battle of Al-Yamama, during the caliphate of Abu Bakr, and it was the day when Musailamah Al-Kadhdhab was killed."