The book Usul al-Sunnah by Imam Ahmad - may God have mercy on him - is considered one of the most important books in defining the belief of Ahl al-Sunnah wal-Jama’ah. - may God’s prayers and peace be upon him - and his companions, and scholars have been so interested in it that Judge Abu Ya’la, may God Almighty have mercy on him, said: “If he were to go to China in pursuit of it, it would have been little.”