Pillars & FiqhThe testimony that there is no god but Allah and that Muhammad is the Messenger of Allah is the first and greatest of the pillars of Islam. Its meaning is to single out Allah alone for worship and to abandon all besides Him, and to affirm the truth of the Prophet ﷺ in what he informed and to obey him in what he commanded. It is the key to Islam, and a servant's Islam is not valid until he utters it, believing in its meaning and acting according to its requirements.
Source:حديث جبريل عن عمر بن الخطاب، رواه مسلم