If the person seeking fatwa happens to be well-versed in comparing different views to each other according to their legal proofs, he does not have to abide by a certain Juristic School. Only what he has to do is to follow the opinion which he finds more convincing. However, if the person is not well-versed in this, then he has to abide by the opinion of a person giving the Fatwa even if they do not belong to the same Juristic School.
As for the divergence in opinions relating to these juristic Schools, it is a difference regarding the evidence cited by each Imam or, you can call it, a difference in a way of understanding each religious text.
Thus, we can describe this evidence as some sort of a blessing in disguise for the course of enlightenment in the sense that it gives Muslim scholars an ample opportunity of understanding Islam better by having in his disposal an assortment of opinions delivered by different Imams based on their understanding of the text. No doubt, this also has a far-reaching effect in the sense that each Muslim society will have an option to choose any opinion suitable for the circumstances that warrant. This is a sign of the flexibility characterizing the Islamic Law.
As for the method applied by each juristic Imam in reaching his opinion based on the evidence deduced from the religious text, they are explained as follows:
The Method of Imam Abu Hanifah:
He resorts to seven sources to arrive at a certain opinion; these sources are:
1- The Glorious Qur’an. 2- The Prophetic Tradition (Sunnah)
3- The sayings of the Prophet’s Companions. As regard this, Imam Abu Hanifah is reported to have said he used to rely on the saying of any Companion of the Prophet regardless of names, though he never sidestepped their opinions for others’. As for the views of Ibrahim, Shu`abi, Ibn Sirin and others, given that they adopted personal reasoning, he used to follow in their footsteps.
4- Qiyas (analogical deduction). 5- Istihsan (Juristic preference). 6- Ijma` (scholarly consensus). 7- `Urf (Custom).
The Method of Imam Malik:
The sources from which Imam Malik deduces the evidence are as follows:
1- The Glorious Qur’an, the Sunnah of the Prophet, and the sayings of the Prophet’s Companions.
2- The custom followed by people of Madinah.
3- Fatwa given by a Companion.
4- Al-Masalih Al-Mursalah (public Interest).
5- Istihsan (juristic preference).
6- Sadd Adh-Dhara’i` (blocking the means to evil)
The Method of Imam Ash-Shafi`i:
He relies on the following sources:
In his book al-Umm (vol. 7), he states that there are various sources of knowledge, first of them is the Glorious Qur’an, then the Sunnah of the Prophet (peace and blessings be upon him). The second is the consensus of Muslim scholars regarding the issue lacking textual proof in the Qur’an or Sunnah. The third is the agreed-upon sayings of some of the Companions regarding the issue in question. The fourth is the controversial views of the Companions. The fifth is the evidence deduced from analogy based on the Qur’an and Sunnah.
The Method of Imam Ahmad:
He relies on five sources:
1- The Glorious Qur’an and Sunnah.
2- Non-controversial fatwas delivered by the Prophet’s Companions.
3- Should they issue any controversial views, the priority is given to the opinion which is in conformity with the Qur’an and Sunnah.
4- Making use of inauthentic and weak hadiths, if there is no other evidence; at least to him this is more preferable than the use of Qiyas.
5- Qiyas: This is the last source for Ibn Hanbal, and it is used only if there is no evidence in the previous sources. And in some cases he adopts cautious approach regarding issues lacking evidence or governed with contradictory evidence even if there are statements uttered by the Prophet (peace and blessings be upon him) and his Companions regarding those issues, but given that those statements are controversial he refrains from issuing fatwa concerning them.
For further information, you can resort to the following references:
1- Al-Madkhal fi al-Fiqh al-Islami by Dr. Abdul-Karim Zeidan
2- Tarikh al-Madhahib al-Islamiyyah by Sheikh Muhammad Abu Zahrah.
3- Usul al-Fiqh by `Abdul-Wahhab Khallaf.