According to Sampiro, Fernando ("Fredenandi Ansuri filius") was one of the counts of the region of Burgos, the chief city of Castile—the others being Nuño Fernández, Abolmondar Albo, and Diego Rodríguez—who were captured by Ordoño II on the river Carrión in the place called Tebulare or Tegulare ("Tejar" or "Tejares" in Spanish, as yet unidentified) and imprisoned them in León.
The later chronicler Pelagius of Oviedo interpolated into Sampiro's account the words et erant ei rebelles ("and they were rebels") in order to explain Ordoño's action, but this is conjecture, as is the modern suggestion that it is related to the defeat at the Battle of Valdejunquera.
The title count is given to the leader of this rebellion by Ibn Hayyan a century later, and it may have been either Fernando or his son Ansur.
Alfonso IV and his allies were defeated in the ensuing civil war, and Ramiro bestowed Castile on his partisan, Fernán González.
The earliest reference to Ansur is found in a document dated 4 March 921, wherein he is named with his parents in a donation to Cardeña in the vicinity of Burgos.