The most important obligation after the Shahadah (Testimony of Faith) is observing the Five Obligatory Daily Prayers. Salah is the cornerstone of Islam. Whoever observes the performance of Salah preserves his religion, and whoever neglects it neglects other obligatory duties.
The mainstay of the matter is Islam, its pillar is Salah, and its topmost part is Jihad (defending the Cause of Allah).
Islam has been built on five (Pillars): testifying that there is no god but Allah and that Muhammad is the Messenger of Allah, establishing the Salah, paying Zakah, fasting Ramadan, and performing Hajj.
The covenant that distinguishes between us and them (the hypocrites) is Salah. Anyone who abandons it is a Kafir.