Kitab al-Futuh
Sayf ibn Umar is considered reliable. W. F. Tucker and Ella Landau-Tasseron note that although Sayf may have been an unscrupulous hadith collector, this should not detract from his general reliability as a transmitter of historical information (akhbārī). Tucker adds that accusations of bias could equally be leveled at other akhbārīs contemporary to Sayf, including the Shi'a historian Abu Mikhnaf. Fuat Sezgin, Albrecht Noth, and Martin Hinds have also challenged Wellhausen's views and placed Sayf on an equal footing with other traditionalists. Linda D. Lau and A. R. Armush also regard Sayf ibn Umar as reliable and accept his accounts and the role of the Saba'iyya at the Battle of the Camel.