Question: In order to become a Muslim, is it enough to say: “Laa ilaaha illaa Allaah (there is no god but Allaah)”, or does it have to be proven by deeds?
Answer: The Shahaadah (Testimony of Faith) is the greatest of all statements. It has to be articulated with the tongue and demonstrated practically. Mere articulation of the Shahadah is not sufficient for a person to be rightfully called a Muslim. Were it sufficient, the hypocrites would be considered Muslims; however, they are still considered Kaafirs (disbelievers) even though they articulate this statement. They will be flung into the depths of the Hellfire. We take refuge with Allaah from this. It is because they utter this statement without developing true faith. Therefore, this statement must be uttered with the tongue and at the same time the heart must firmly believe that there is no deity truly worthy of worship except Allaah. One must practically demonstrate the Shahaadah by fulfilling the duties prescribed by Allaah (Exalted be He) and avoiding what He has prohibited. These are essential requirements of Shahaadah. This is based on the Hadeeth in which the Prophet (peace be upon him) is reported to have said: I have been commanded to fight against people (i.e disbelievers) until they bear witness that there is no god but Allaah. Should they utter it, their blood and property will become inviolable unless they commit a sin which is punishable under Islaam, and their reckoning rests with Allaah. In another Hadeeth, the Prophet (peace be upon him) is reported to have said… read more here.