Khalid ibn al-Walid was one of the finest commanders of the Quraysh. At Uhud he led the idolaters' cavalry. But after the Treaty of Hudaybiyyah he came to Madinah and accepted Islam.
In the eighth year after the Hijrah, the Muslim army faced a vast Byzantine force at Mu'tah. The Messenger of Allah ﷺ had named three commanders in turn, and all three were martyred. Then Khalid took the banner.
❝ HadithAnas narrated that before the news arrived, the Messenger of Allah ﷺ told the Companions in Madinah: Zayd took the banner and was martyred, then Ja'far took it and was martyred, then Ibn Rawahah took it and was martyred — his eyes were streaming with tears — until one of the swords of Allah took the banner, and Allah gave them victory.Sahih al-Bukhari 3757 · Sahih
From that day Khalid was known as Sayfullah — the Sword of Allah.