According to the 10th century Muwallad historian Ibn al-Qutiyya, Count Cassius converted to Islam in 714 as the mawali (client) of the Umayyads, shortly after the Umayyad conquest of Hispania, and his family came to be called the Banu Qasi (sons or descendants of Cassius). Cassius had converted at the hands of the Arab, Hassan ibn Yassar al-Hudhali, qadi in Zaragossa at the time of Abd ar-Rahman's arrival in the peninsula. He converted to Islam as a means to preserve his lands and political power. Cassius joined forces with Musa ibn Nusayr and Tariq ibn Ziyad and travelled to Damascus to personally swear allegiance to the Caliph Al-Walid I.
Another Arab historian Ibn Hazm who lived in the 11th century, listed his sons as Fortun, Abu Tawr, Abu Salama, Yunus and Yahya. The Banu Qasi dynasty was directly descended from Fortun, the eldest son of Count Cassius, while it has been suggested that the second son may be the Abu Taur of Huesca who invited Charlemagne to Zaragoza in 778. It has further been suggested that the Banu Salama, a family that ruled Huesca and Barbitanya (Barbastro) in the late 10th century, may descend from Abu Salama.
However, there is a certain degree of doubt among some historians as to whether Count Cassius ever existed, partly because the name Cassius is not attested to anywhere in the period as a name. They point out that the origins of the Banu Qasi, as recounted by Ibn al-Qutiyya, could be a product of the spurious antiquarianism of the latter Umayyad period rather than reliable genealogy, that satisfied the need for stories which bridged the conquest.