All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
It is permissible to give Muslim children the names “Sayfullaah” or “Saifan” if it means “the sword of Allaah” in your language, and the same ruling applies to the names indicating similar meanings such as Muhannad (sword). The name Sayfullaah is one of the good names and nicknames (acceptable in Islam).
Anas reported that the Prophet, sallallaahu ʻalayhi wa sallam, said in the story of the Battle of Mu'tah, “Then the banner was taken up by a Sword from the Swords of Allaah (referring to Khaalid ibn Al-Waleed) and Allaah made them (i.e. the Muslims) victorious.” [Al-Bukhari]
Al-Haafith Ibn Hajar wrote, “In the narration reported on the authority of Abu Qataadah: From this day forward, Khaalid ibn Al-Waleed was nicknamed Saifullaah (the Sword of Allaah).” [Fat-h Al-Baari]
Abu ʻUbaydah reported that the Prophet, sallallaahu ʻalayhi wa sallam, said, “What an excellent slave of Allaah is Khaalid ibn Al-Waleed, a sword from among the swords of Allaah.” [Ahmad]
Shaykhul-Islam Ibn Taymiyyah wrote, “The nickname 'Sword of Allaah' is not particular to Khaalid ibn Al-Waleed; rather, he was one of the swords of Allaah unleashed against the disbelievers. This was indicated in the hadeeth. The Prophet, sallallaahu ʻalayhi wa sallam, was the first to call him with this nickname...” [Minhaaj As-Sunnah]
For further benefit, please refer to fataawa 82724 and 87262.
Allaah knows best.