He was referred to as Abū ʿAbdillāh Fayrūz al-Daylamī, a descendant of the Persians who were sent by Khusraw to the Yemen, conquered it, and drove out the Abyssinians. According to ʿAbd al-Munʿim: They later traced their pedigree to the Children of Dabbah, saying “We had been taken prisoner in pre-Islamic times.” ʿAbd al-Munʿim was mistaken in what he said, for [the story] was like this: Dabbah ibn ʿUd had three sons. One of them assaulted another descendant of Dabbah and killed him. His father wanted to kill him [in retaliation], so he ran away and settled in the Daylam mountains, where he fathered several children.
His descendants claim even today that they have in their possession his saddle and utensils. Fayrūz is the one who killed al-ʿAnsi, that is, al-Aswad b. Kaʿb al-Kathāb (the liar), who claimed to be a prophet in the Yemen. The Prophet said ﷺ “He was killed by the virtuous man – Fayrūz al-Daylamī.” It has been reported that Fayrūz al-Daylamī killed him while al-Aswad laid in his bed. Fayrūz said; “News reached the Prophet ﷺ from the heavens the very same night that he was killed“. “Al-Aswad has been killed by a righteous man” said the Prophet ﷺ, ‘Who killed him?” the Companions asked, ‘Fayrūz! Fayrūz has been victorious!‘ replied the Prophet ﷺ.
He was bestow with a son named ʿAbdullah who was from the senior scholars of the successes. Shaykh Ṣāliḥ b. al-Fawzān said; ‘ʿAbdullah was the son of Fayrūz ad-Daylamī, a trustworthy senior scholar of those who came after the Companions. His father, Fayrūz, killed al-Aswad al-ʿAnsī, the Liar.’
Fayrūz died in Yemen in the year 53AH