Taqlid in the Qur'an and Ahadith

Taqlīd is an Islamic jurisprudential term which means to follow a Mujtahid in practical rulings of the religion. Taqild is not permissible in Usul al-Din (Principles of the Religion) and one must accept them by research, study and inquiry. However, in Ahkam (Islamic Rulings) one can be either Mujtahid or act according to Ihtiyat (caution) or follow an expert in Islamic jurisprudence - who is called Mujtahid.

Taqlid in the Quran

The Qur'an instructs Muslims to seek guidance from people of learning in matters about which they lack knowledge:

…so ask the people of the message if you do not know. (21:7) [1]

It is an obligation in Islamic law to study everything which is necessary for the spiritual and material development and well-being of an Islamic community, but it is an obligation which is known as wajib kifa'i.[2]
In the present instance, for example, an Islamic society has need of experts in the medical sciences, in physics and chemistry, engineering, education, and so forth, and as long as there is a lack of knowledge in these areas it is an obligation on the community as a whole to acquire it, which means that a group of Muslims should devote themselves to research so as to benefit the Islamic people as a whole.
Similarly, an Islamic society without experts in the shari'ah cannot properly consider itself Islamic, so it is an obligation for a group of persons from this society to devote themselves to the study of the religious sciences, so as to provide divine guidance for all Muslims. This is the meaning contained in the verse of the Qur'an which states:

"But why should not a party from every section of them (the believers) go forth to become learned in the religion, and to warn their people when they return to them, that they may beware?" (9:122) [3]

Taqlid in Ahadith

In a famous hadith, `Umar ibn Hanzalah asked Imam Ja`far as-Sadiq, peace be upon him, about the legality of two Shi'ahs seeking a verdict from an illegitimate ruler in a dispute over a debt or a legacy. The Imam's answer was that it was absolutely forbidden to do so.
Then Ibn Hanzalah asked what the two should do, and the Imam replied:

"They must seek out one of you who narrates our traditions, who is versed in what is permissible and what is forbidden, who is well-acquainted with our laws and ordinances, and accept him as judge and arbiter, for I appoint him as judge over you. If the ruling which he based on our laws is rejected, this rejection will be tantamount to ignoring the order of Allah and rejecting us is the same as rejecting Allah, and this is the same as polytheism."[4]

In another tradition from Imam Ja'far as-Sadiq, this time narrated by Imam Hasan al-`Askari, peace be upon them, he says,

"...but if there is anyone among the fuqaha' who is in control over his own self, protects his religion, suppresses his evil desires and is obedient to the commands of his Master, then the people should follow him."[5]

A third hadith is from the Present Imam, Muhammad al-Mahdi, peace be upon him, who said in a reply to Ishaq ibn Ya'qub:

"As far as newly occurring circumstances are concerned, you should turn (for guidance) to the narrators of our ahadith, for they are my proof over you just as I am Allah's proof."[6]

We can understand two things from these verses of the Qur'an and the ahadith of the Imams:
1) there must always be a group of fuqaha' in every Muslim society;
2) those who are not qualified as fuqaha' or mujtahids, must follow one, and that to go against his instruction in religious matters is tantamount to polytheism..

Note:
[1] فَاسْأَلُوا أَهْلَ الذِّكْرِ‌ إِن كُنتُمْ لَا تَعْلَمُونَ
[2] Wajib kifa'i = the obligation which is on every member of the community as long as it is unfulfilled, but as soon as some person, or persons, has fulfilled it, it is no longer an obligation on those who have not fulfilled it.
[3] وَمَا كَانَ الْمُؤْمِنُونَ لِيَنفِرُ‌وا كَافَّةً  فَلَوْلَا نَفَرَ‌ مِن كُلِّ فِرْ‌قَةٍ مِّنْهُمْ طَائِفَةٌ لِّيَتَفَقَّهُوا فِي الدِّينِ وَلِيُنذِرُ‌وا قَوْمَهُمْ إِذَا رَ‌جَعُوا إِلَيْهِمْ لَعَلَّهُمْ يَحْذَرُ‌ونَ
[4] Shaykh al-Kulayni, Furu`u 'l-Kafi, vol. 7, Tehran 1379, p. 412.
قَالَ: سَأَلْتُ أَبَا عَبْدِ اللَهِ عَلَیْهِ السَّلا مُ: عَنْ رَجُلَیْنِ مِنْ أَصْحَابِنَا یَکُونُ بَیْنَهُمَا مُنَازَعَةٌ فِی‌ دَیْنٍ أَوْ مِیرَاثٍ، فَتَحَا کَمَا إلَی‌ السُّلْطَانِ أَوْ إلَی‌ الْقُضَاةِ، أَیَحِلُّ ذَلِکَ؟فَقَالَ: مَنْ تَحَاکَمَ إلَی‌ الطَّاغُوتِ فَحَکَمَ لَهُ، فَإنَّمَا یَأْخُذُ سُحْتًا، وَ إنْ کَانَ حَقُّهُ ثَابِتًا. لاِنـَّهُ أَخَذَ بِحُکْمِ الطَّاغُوتِ وَ قَدْ أَمَرَ اللَهُ أَنْ یُکْفَرَ بِهِ.  قُلْتُ: کَیْفَ یَصْنَعَانِ؟  قَالَ: انْظُرُوا إلَی‌ مَنْ کَانَ مِنْکُمْ قَدْ رَوَی‌ حَدِیثَنَا وَ نَظَرَ فِی‌ حَلا لِنَا وَ حَرَامِنَا، وَ عَرَفَ أَحْکَامَنَا، فَارْضَوْا بِهِ حَکَمًا فَإنِّی‌ قَدْ جَعَلْتُهُ عَلَیْکُمْ حَاکِمًا، فَإذَا حَکَمَ بِحُکْمِنَا فَلَمْ یَقْبَلْهُ مِنْهُ، فَإنَّمَا بِحُکْمِ اللَهِ قَدِ اسْتَخَفَّ، وَ عَلَیْنَا رَدَّ، وَ الرَّادُّ عَلَیْنَا الرَّادُّ عَلَی‌ اللَهِ؛ وَ هُوَ عَلَی‌ حَدِّ الشِّرْکِ بِاللَهِ
[5] مَن كانَ مِن الفُقَهاءِ صائنا لنفسِهِ حافِظا لِدينِهِ مُخالِفا على هَواهُ مُطِيعا لأمرِ مَولاهُ فلِلعَوامِّ أن يُقَلِّدُوهُ
[5] Shaykh at-Tabarsi, al-Ihtijaj, vo. 2, Najaf 1966, p. 263.
[6] Kamal al'Din va tamam al'Nema. V 2. P493.
اما الحوادث الواقعه فارجعوا فيها إلى رواة حديثنا فانّهم حجّتى عليكم و انا حجّة اللّه.
[7] For these and further rulings concerning the marja'u 't-taqlid see Sayyid Muhammad Kazim at-Tabataba'i al-Yazdi, al-`Urwatu'l-Wuthqa', Tehran 1972 (published with the annotations of the contemporary maraji'), p. 5.