The book of The Major Classes (Arabic: Kitab Tabaqat Al-Kubra) was is a compendium of biographical information about famous Islamic personalities. It is eight volumes long. This work contains the lives of the Prophet Muhammad (pbuh), his Companions and Helpers, including those who fought at the Battle of Badr as a special class, and of the following generation, the Followers, who received their traditions from the Companions. Ibn Saad's authorship of this work is attested in a postscript to the book added by a later writer. In this notice he is described as a "client of al-Husayn ibn Abdullah of the Abbasid family". (This is according to Wikipedia)