Answered by Shaykh Hamza Karamali, SunniPath Academy Teacher
In the Name of Allah, Most Merciful and Compassionate
Our purpose, as the Qur'an explains, is to worship Allah. Allah created us on earth and commanded us to do certain actions and refrain from others. In order to obey Him, we need to know what He demands of us. Hence the importance of sacred knowledge.
In terms of fiqh (knowledge of the rulings of sacred law), Muslims fall into one of two categories. They are either mujtahids (loosely meaning that they possess the stringent qualifications required to derive rulings directly from the Qur'an and Sunna) or muqallids (meaning that they do not possess these qualifications and hence must resort to a mujtahid to find out Allah's ruling on a particular matter). No Muslim will contest that we must follow the Qur'an and Sunna; the Question rather is: is one qualified to derive legal rulings oneself, or must one submit to those who are authorized to do so?
Mujtahids are of various kinds (one can refer to Kamali's "Principles of Islamic Jurisprudence" for a categorization of the different levels of ijtihad) . Only the highest level of mujtahid (the absolute (mutlaq) mujtahid) is not bound to a madhab, and Imam Nawawi mentions in the Majmu' that this mujtahid has not existed since the era of the mujtahid imams. The only ijtihad that is probably alive today is on new issues that were not addressed by the classical works of the madhabs.
Since a mujtahid can derive fiqh rulings directly from the primary sources (commonly enumerated as four: Qur'an, Sunna, Ijma', and Qiyas, although there are others as well), it is haram for him to make taqleed. For him to do so would be to turn away from the Qur'an and Sunna and follow the opinions of men. The verse is Surah al-Tawba where Allah condemns the Jews for taking their Rabbis as lords besides Allah would apply to a mujtahid in such a case.
The concept of authority is not a difficult one to understand. No society would allow someone to perform a surgical operation without first attending medical school; nor would it permit anyone to build a bridge without first graduating from a civil engineering program. Does it not make sense to define criteria for mujtahids to prevent ignoramuses from ruining the Muslims' afterlife?
It is haram for a muqallid to attempt ijtihad and he is sinful for doing so even if he ends up coming to the right conclusion.  Allah's laws are not something to be played with and only those qualified to perform ijtihad have permission to delve into the primary evidences and deduce that something is halal or haram. The unqualified must ask the qualified. A mujtahid is to a muqallid what the Qur'an and Sunna are to a mujtahid. To find out the ruling of Allah on a particular issue, we resort to the fiqh books of the four schools; not to Sahih al-Bukhari or Sahih Muslim (let alone the would-be mujtahids of our age). This is not because we don't want to follow the Qur'an and Sunna; rather, it's because the only way we can truly follow the Qur'an and Sunna is by going through someone who understands what they are saying.
If a mujtahid looks at the prayer of a muqallid and rules that his prayer is valid, the muqallid has absolved himself of his responsibility before Allah, regardless of any hadiths that Salafis or modernists might hit him on the head with. Allah has demanded that the muqallid ask the mujtahid, saying, "Ask those who know well if you know not." Those who obstinately shunt aside the madhabs and follow what they think is right risk falling under the purview of the Prophetic curse: "They killed him; may Allah kill them: why did they not ask when they did not know?" 
This is where talfiq comes into the picture. Talfiq is to join between the positions of more than one school so that the resulting amalgam would be unacceptable according to all mujtahids. For example, someone who bleeds and decides to follow the Shafi'i school on wudu cannot follow the Hanafi school in prayer by not reciting the fatiha behind the imam, since both schools would say that his prayer was invalid. A muqallid who does talfiq in this manner has not absolved himself of his responsibility before Allah because there is no mujtahid who would look at his action and say that he had done what Allah asked of him.
With this in mind, Imam Ibn Hajar al-Haytami's (whose word, along with Imam Muhammad al-Ramli's, constitutes the final word in terms of fatwa in the Shafi'i school) definition of talfiq makes perfect sense. He said: "Whoever makes taqleed of an Imam in an issue must follow the requirements of his madhab on that issue and everything else that is related to it." (I'atu'l-talibin, 4.219). Since wudu and salat are related (wudu is a condition for the prayer), one must follow the same school on both. Since zakat, fasting, pilgrimage and prayer are not related to each other, one can validly follow a different school in each one.
Mixing between Schools without Talfiq
Since a muqallid's responsibility is only to fulfill the requirements set out by any of the madhabs, one could conceivably follow the positions of mutliple schools in a manner that avoids talfiq, praying fajr as a Hanafi, zuhr as a Shafi'i, asr as a Hanbali, and maghrib as a Maliki, and one would have absolved oneself of one's responsibility to perform these four prayers. The option to follow an opinion from another school can be useful when following the position of one's own madhab would lead to undue hardship, which is why the scholars mention that differences of opinion are a mercy.
The 'ulama also mention, however, (among them Ibn Hajar in the same reference given above), that someone who follows positions in a different school without need and only to seek the easy way out is sinful and thereby becomes a fasiq (corrupt). This is not something to be taken lightly, and is particularly relevant these days because of the ease with which one can find out the opinion of another madhab on a certain issue. Many of us might be subscribed to the Hanafi list, the Shafi'i list and the Hanbali list and seeing easy rulings in another madhab might seem tempting at times. To seek out easy rulings from a madhab other than the one one usually follows without any pressing need would be sinful.
As a final note, those who haven't already done so should read the articles on madhabs, taqlid, and ijtihad on Mas'ud Khan's webpage (www.masud.co.uk) by Shaykh Hamza Yusuf, Shaykh 'Abd al-Hakim Murad, and Shaykh Nuh Keller. These and other articles on this website are essential reading for all Sunni Muslims in these troubled times.
And Allah knows best.
 Although Kamali's book is one of the best references on usul al-fiqh available in English, the author relies mainly on some modern Arabic works on the topic and many of these works have modernist inclinations that go against classical Islamic scholarship. This is particularly noticeable in some contentious positions taken by the author in the section on ijtihad. So the book needs to be read with a grain of salt.
 The Prophet (Allah bless him and give him peace) said, "Judges are of three types: one is in Paradise and two are in the Fire. As for the one who is in Paradise , he is the one who knows the truth and judges according to it. The one who knows the truth and wrongs others by ruling [against the truth] is in the Fire. The one who rules while being ignorant is [also] in the Fire." (Abu Dawud, Tirmidhi, Ibn Majah). 'Allamah al-Sindi (Allah have mercy on him) said in his commentary on Sunan Ibn Majah: "The generality of the wording ["The one who rules while being ignorant"] includes the case when he judges correctly. This is because he deserves the Fire by daring to take up such an action without knowledge."
 Modernists and reformists insist that this verse was revealed in a certain context. Such objections, however, are indicative of their lack of knowledge of usul al-fiqh: a science that must be mastered before attempting to interpret any of the primary texts. Texts of usul tell us that general statements made because of specific reasons are not confined to the specific reasons but rather remain generally applicable to all situations (al-'ibratu bi 'umum al-lafdh la bi khusus al-sabab). Someone who doesn't understand this point needs to study usul before raising any objections. Regardless, the point the ayah is making is obvious even without it.
 Abu Dawud, Ibn Majah, Ahmad, and Darimi relate a hadith about a man whose head was split by a rock while travelling. This man had a wet dream and then asked the companions around him if he could take the dispensation of tayammum instead of performing a full ghusl. The companions insisted that he should use water. He made ghusl with water and then died because of it. When the Prophet (Allah bless him and give him peace) heard about this, he prayed against them by saying, "They killed him; may Allah kill them. Why didn't they ask if they didn't know? The only cure for ignorance is to ask. It would have sufficed him to make tayammum, wipe over his wound, and wash the rest of his body."
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