This important work is a source-critical study of a group of traditions (aḥādīth) found in Ibn Isḥāq's Biography (Sīra) of the prophet Muḥammad, widely considered one of the most important early historical texts on the Prophet's life. Through a meticulous isnād-cum-matn analysis, the author reveals that Ibn Isḥāq relied on Muḥammad b. Abī Muḥammad, a hitherto undocumented source of his. Important new light is also shed on problems with Ibn Hishām's recension of Ibn Isḥāq's Sīra.