Question: What are the requirements of ‘Laa ilaaha illaa Allaah (there is no god but Allaah)’?
Answer: This phrase is the greatest speech ever. It is the word of Tawheed (monotheism). A person cannot be called a Muslim unless they first pronounce this phrase and firmly believe in it. They must believe that there is no god worthy of worship except Allaah (Exalted be He). This was the first word which all the messengers preached to people: ‘Laa ilaaha illaa Allaah (there is no god but Allaah)’.
Allaah (Exalted be He) says, And We did not send any Messenger before you (O Muhammad صلى الله عليه وسلم) but We revealed to him (saying): La ilaaha illaa Ana [none has the right to be worshipped but I (Allaah)], so worship Me (Alone and none else).” He (Glorified be He) also says, And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming): “Worship Allaah (Alone), and avoid (or keep away from) Taaghoot (all false deities i.e. do not worship Taaghoot besides Allaah).”
The first word to which our Prophet Muhammad (peace be upon him) invited people was ‘there is no god but Allaah’, together with inviting them to believe that he was the Messenger of Allaah … read more here.