He was al-Tabari's principal source for the early Arab conquests and the history of the Caliphate down to the death of `Ali. The great German scholar, Julius Wellhausen (1844-1918) wrote a devastating critique of Sayf's work in his Prolegomena zur altesten Geschichte des Islams (1899), and Sayf's reputation has not been high ever since. Wellhausen places Sayf in what he calls the "artificial" tradition of Arab historical writing, which was full of "tendentious distortions and fictitious tales invented for their literary impact."