The shahadah (الشهادة), or the Islamic creed, is the declaration of belief in the unity of God (Allah in Arabic) and in Muhammad صلى الله عليه و سلم as his prophet. Recitation of the shahadah is considered as one of the Five Pillars of Islam. When sincerely stated aloud, one is considered to have officially declared oneself a Muslim.

Arabic text:
لا إله إلا الله محمد رسول الله

Reorganization:
"la 'ilaha 'illal-lahu Muhammadur rasulu llahi."

English translations:
‘There in no Allah, but Allah; Muhammad is the messenger of Allah’

Honest recitation of the shahadah three times, in Arabic, in front of two Muslim witnesses, is all that is required for a person to become a Muslim according to most traditional schools.

Sometimes أشهد أن (ašhadu 'an)"I testify that" is prefixed to each half of the Shahada.

History

One of the earliest surviving translations of the Shahadah into a foreign language is in Greek, from the reign of Al-Walid ibn Abd al-Malik (86-96 AH, 705-715 CE).
“Ouk estin theos ei me ho theos monos; Maamet apostolos theou"
"There is no god if not the single god; Muhammad is god's apostle", i.e. Allah is being translated as "the single god".