It is related from Imam Sadiq in an authenticated tradition:
Taqiyyah in mu din and the din of mu forefathers.
Whosoever has no taqiyyah has no din
It was the motto of the Household of the prophets, so as to protect themselves and their followers from harm and bloodshed, and to better the condition of the Muslims and to cause agreement among them, and restore them to order.
3 And this is still a sign by which the Shi'a are known, and which distinguishes them from other sects and other peoples, Everyone, when he feels that there is danger to himself or to his property through the preaching of his beliefs or through the practicing of his beliefs in public, should practice taqiyyah. And this is something which appears reasonable to our natural intelligence, It is known that the Shi'a and their Imams have suffered much and have been denied their freedom throughout history, and that no sect or people have suffered like them. thus they have been forced on many occasions to practice taqiyyah in order to conceal themselves from those with other beliefs; to keep themselves and their practices hidden, as long as their religion and their survival was threatened. And this is their cause for being distinguished from others by their taqiyyah.
Taqiyyah has rules and observations which indicate whether it is obligatory (wajib) or not, and these are mention in the relevant chapter of the books of those learned in jurisprudence (fiqh). It is not obligatory at all times, but is sometimes optional; and sometimes it is obligatory not to doe it, as when it is necessary to proclaim the truth publicly, to protect Islam and save it, or to fight in the cause of Islam. On these occasions, property is of no value, and individual souls are of no importance. Taqiyyah is forbidden (haram) when someone's life is in danger, or when falsehood is being propagated, or when anything is threatening Islam, or when Muslims are menaced, or injustice and iniquity are spreading among them. The purpose of taqiyyah, in the view of the Shi'a in not to form a secret organization dedicated to destruction and subversion, as some of their enemies, who are not able to see things in there true light, have imagined, for such people have made no effort to really understand what we say. The point is not to make Islam and its rules a secret which cannot be divulged to those who do not believe. No, the books of Shi'a and their writings in the fields of jurisprudence (fiqh), law (ahkam) and theological studies, as also their beliefs, are in great abundance in the world, more than any other sect that is sure of its way. Our belief in taqiyyah has been abused by those who want to degenerate the Shi'a, and they consider it to be one of their weak points, for its seems that they are not satisfied with the necks that fell to the sword in the attempt to finish them off in that age when it was enough to say that someone was a shi'a to assure his death at the hands of the enemies of the Household of the Prophet, from the Umayyids and the' Abbasids up to the Ottomans.
If our attacks wish to say that there is not evidence for taqiyyah in Islam, we can refute this. Firstly we fellow our Imams and are guided by their guidance, and they have ordered us to practise tayqiyyah when it is necessary, and it is to them an integral part of Islam, as we saw from the saying of Imam Sadiq:
Whosoever has no taqiyyah has no din.
Secondly, it has been commanded in the Qur'an:
Not he who is compelled while his heart is at rest on account of his faith. (16;106)
This verse was revealed about 'Ammar ibn Yasir, who took shelter by proclaiming unbelief in order to protect himself from the enemies of Islam.
Also Allah has said:
And a believing man of Pharaoh's people who hid his faith ... (40;28)
