All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The pillars of Islam have been clearly outlined by the Prophet, sallallaahu ‘alayhi wa sallam , in the famous Hadeeth of Jibreel (Gabriel) which was narrated on the authority of ‘Umar Ibn Al-Khattaab . In this Hadeeth, the Prophet, sallallaahu ‘alayhi wa sallam, clarified that Islam is built on five pillars: the declaration of the Two Testimonies of Faith, performing the obligatory prayers, paying the obligatory Zakah (alms), the fasting of Ramadan and the performance of Hajj for whoever can afford it. [Muslim]
These rules have been illustrated in books that discuss the Islamic creed and Tawheed (monotheism), which have also thrown light on the Muslim’s duty towards these rules. This was previously clarified in detail in Fatwa 8577.
Moreover, it should be known that the word "Islam", in general, refers to all the three levels of religion: Islam (submission), Eemaan (faith) and Ihsaan (sincerity in worship).
For information on the Islamic ruling on the one who gives up prayer, see Fatwa 1145.
For information on the Islamic ruling on abandoning acts of worship altogether, see Fatwa 17836.
Allaah Knows best.