Ya'qub ibn Ibrahim al-Ansari (), better known as Abu Yusuf () (d.798) was a student of jurist Abu Hanifa (d.767) who helped spread the influence of the Hanafi school of Islamic law through his writings and the government positions that he held. He served as the chief judge (qadi al-qudat) during reign of Harun al-Rashid. Kitab al-Kharaj, a treatise on taxation and fiscal issues facing the state, is his most well-known publication. Biography Abu Yusuf lived in Kufa and Baghdad, in what is now Iraq, during the 8th century. His genealogy has been traced back to Sa'd ibn Habta, a youth in Medina in the time of the Prophet, and his birth date is estimated based on the date of his death to be around 113/729CE. Based on anecdotal stories, Abu Yusuf was raised poor but with a ferocious appetite for knowledge. His mother disapproved of his academic desires, insisting that he master some trade (the art of tailoring, according to some source) so as to help make ends meet. While it cannot be fully verified, stories suggest that he complied with his mother's wishes, but also kept up his academic studies. His talent and commitment was eventually recognized by Abu Hanifa who became his mentor with Abu Yusuf as his star pupil. He is portrayed as an incredibly studious individual who was unceasing in his pursuit for knowledge and legal understanding. While much of what is known of his early childhood relies on sometimes contradictory anecdotal evidence, it has been verified that he studied religious law and traditions in Kufa and Medina under a number of scholars including Abu Hanifa, Malik b. Anas, al-Layth b. Sa'd and others. Under the guidance of Abu Hanifa, Abu Yusuf achieved incredible success and helped develop and spread the influence of the Hanafi school of Islamic law. Abu Yusuf lived in Kufa until he was appointed Qadi in Baghdad. It is unclear whether he was appointed by Mahdi, al-Hadi, or Harun al-Rashid. According to one story, Abu Yusuf was able to provide sound advice pertaining to religious law to a government official who rewarded him generously and recommended him to the caliph, Harun al-Rashid. He continued to provide satisfactory legal opinions to the caliph who drew him into his inner circle and eventually appointed him Qadi. While this version of events is probable, it is not necessarily authentic and cannot be independently verified.