"Milestones on the way"
Author: Said Qutb.
Excerpts from Milestones on the Path
The call "There is no god but Allah" is the law of life.
Worshiping Allah, and only Him, is the cornerstone of the Islamic faith, embodied in the testimony: "There is no god but Allah." The perception of how [exactly] to worship from the Messenger of Allah (peace and blessings of Allaah be upon him) is the second component, found expression in the statement that Muhammad (peace and blessings of Allaah be upon him) is the Messenger of Allah.
This rule with its two parts is assimilated by the heart of a devout Muslim. Everything that follows these two parts [shahada], which constitutes the basis of the Islamic faith, is nothing but a derivative conditioned by this evidence. Belief in the angels of Allah, in His Scriptures, in His messengers, the last day and predestination with its good and evil, as well as prayer, zakat, fasting and Hajj, any prohibitions and expression of condemnation, permissiveness, holiness [of this or that action], legal regulations, statutes and Islamic precepts are all based on one thing: the worship of Allah, and only Him. The source of all this is what the Messenger of Allah (peace and blessings of Allaah be upon him) told us from his Lord [Holy Quran] ...