Ar: to bear witness, to confess [that there is no god but Allah and that Muhammad is His Messenger].
Confession of faith, the first pillar of Islam, without which all the other pillars are of no value. This formula is to be repeated: "Ashhadu an la ilaha illa 'llah; ashhadu anna Muhammadan rasulu 'llah" : "I witness that there is no god but Allah, and Muhammad is the messenger of Allah." Taken from Muhammad 47:19 and al-Fath 48:29.
The importance of the Shahada to a Muslim is not only in what it says, but that by reciting it, a non-Muslim is converted to Islam. Muslim teachers, however, say that more than simply reciting these words in Arabic is necessary. Their meaning must be believed in the heart as well. Once the Shahada has been professed, no one can declare the confessor a non-Muslim, no matter how evil an act he may later happen to commit. Muslims believed that the first part is the same for all prophets, but that the second is changed to whichever the prophet is, ie. Moses: "There is no god but Allah, and Moses is the Converser with Allah"; Jesus: "There is no god but Allah, and Jesus is the Spirit of Allah," etc.
Many Muslims utter the Shahada in the ears of the newly-born infants, and is the last word of the dying and whispered into their ears when the power of speech is gone. At the Muslim graveyard, all repeat it in unision.
Shahada is mentioned after "Allahu Akbar" and at the end of prayer. It is done with first finger of right hand extended as a witness to the unity of Allah.
In the Muslim Brotherhood, they repeat prayer for forgiveness 100 times, prayer for Muhammad 100 times, and Shahada 100 times. Repeating the Shahada is believed to be an act of piety.
One tradition says that Hell fire will not touch anyone who has repeated "la ilaha illa 'llah". Another says that by repeating the Shahada 100 times, a man will "be raised up by Allah on the Day of Resurrection with a face shining like the moon on the night of its fullness." Another has Muhammad saying: "He who dies while acknowledging that there is no deity save Allah, and holds fast to his belief in it, will enter Paradise." Another has Muhammad saying: "A man will be brought along to the Balance on the Day, and there will be brought out for him 99 scrolls, every scroll of them stretching as far as the eye can reach, in which are written his sins and his guilty acts, and they will be put in one pan of the Balances. Then a piece of paper will be brought out about the size of an ant, on which is written the confession, 'The is no deity but Allah, and Muhammad is his servant and his apostle,' which will be put in the other pan, and it will outweigh all his wrong doings." (traditions from Khazinat al-Asrar, Cairo, 1343 AH, see also [*, *, *, *])
Compare with Deuteronomy 4:35; 6:4, Isaiah 44:6; 45:21, Jeremiah 23:24, Mark 12:29-31, John 17:3.
* Some Muslims, however, view the second part of the Shahada as shirk, since, they reasoned, it associates Muhammad with Allah. They only recite the first part of this formula.
Further reading:
The True Shahada (on John 17:3)
Sunni Islam's Real Shahadah