The Prophet (peace be upon him) said: &"Islam is built on five (pillars): The testimony that there is none worthy of worship but Allah, establishing Salah, giving Zakah, Hajj, and fasting Ramadan&" (Narrated by Al-Bukhari)
&"La ilaha illallah&" for its sake Allah beautified Jannah and He created Jahanem, and for its sake the scale of good and evil deeds will be set up.
Furthermore he (peace be upon him) said: &"The best of what I and the Prophet before me have said is: &"None has the right to be worshipped but Allah, Alone. He has no partner, to Him belongs all that exists, and to Him belongs the Praise, and He is over all things (in His power).&" (La ilaha illallah, wahdahu la sharika Lahu, Lahul-Mulku wa Lahul-Hamdu, wa Huwa ala kulli shai’in qadir)’” (Narrated by At-Tirmidthi)
He (peace be upon him) also said: “When prophet Nuh was dying, he told his son: ‘I will give you some instructions. I will command you to do two things and I will forbid you from doing two things. I command you to say: “
There is no god but Allah.” If the seven heavens and the seven earths were a dark ring, they would be broken to pieces by this phrase: &"There is no god but Allah&"&"(Narrated by Al-Bukhari in “Al-Adab Al-Mufrad”)
source:
Islamkingdom