The Shahadah is the Muslim declaration of faith, and one of the Five Pillars of Islam. When saying the shahadah, Muslims are declaring belief in the oneness of God and acceptance of Muhammad as God's prophet.
The Sunni declaration reads: لا إله إلا الله محمد رسول الله There is no god but God, and Muhammad is the messenger of God.
A single honest recitation of the Shahadah in Arabic is all that is required for a person to become a Muslim. This declaration, or statement of faith, is called the Kalima, literally "word". Recitation of the Shahadah, the "oath" or "testimony", is the most important of the Five Pillars of Islam for Muslims. Non-Muslims wishing to convert to Islam do so by a public recitation of this creed. Technically the Shi'a do not consider the Shahadah to be a separate pillar, but connect it to the Aqidah. The complete Shahadah cannot be found in the Quran, but comes from hadiths.