This is an extract from the book Al-Waadih Fee Usool ul-Fiqh by Muhammad Hussein Abdullah.
The meaning of Al-Ijmaa’:
It is the determination or resolve upon a matter and agreement. This resolve is valid to emanate from one (person) and this meaning is found in the Qawl of the Messenger (saw):
مَنْ لََْ يََْمَعْ ال ي صِّيَامَ قَ بْلَ الْفَجْير فَلَ ي صيَامَ لَه “Whoever does not resolve (i.e. intend) the fasting before Fajr, then there is no fasting for him” (related by the compilers of the Sunan).
Just as it is valid to be used for more than one like in His Qawl Ta’Aalaa:
فَأَ يْ جَعُوا أَمْرَكُمْ وَشُرَكَاءكَُمْ
So resolve upon your plan along with your associates (Younus 71).
Which means resolve whilst being in agreement with your associates.
The meaning of Al-Ijmaa’ in accordance to the Istilaah of the Usooliyeen:
It is the agreement upon the Hukm of a reality from amongst the realities in respect to it being a Hukm Shar’iy after the passing of the Messenger of Allah (saw).
The Importance and Significance of Al-Ijmaa’ within the Ahkaam Ash-Shar’iyah
1 – Al-Ijmaa’ is the third Daleel after the Qur’aan and the Sunnah because it based upon a Daleel Shar’iy even though that Daleel did not reach us (directly) but rather only the Hukm deduced from it has reached us.
2 – The ‘Ulamaa of the Muslims have made Ijmaa’ (held a consensus) upon Al-Ijmaa’ representing a Hujjah (proof and evidence) even if they have differed in respect to those who are the people of Ijmaa’ and whose agreement indicates and guides to the convening of Al-Ijmaa’ (a consensus).
3 – The Fuqahaa dedicated special attention and care to identify and acquire knowledge of the areas and contexts of Ijmaa’ amongst the Sahaabah (rah) so as to follow them within them. This is whilst every Mujtahid dedicated special care and attention do not go outside of that which the Sahaabah (rah) had agreed and held an Ijmaa’ upon.
4 - The Hukm that has been established by Al-Ijmaa’ is not abrogated because it came after the death of the Messenger of Allah (saw) and after the Wahi had ceased to descend.
5 – The Sahaabah (rah) have transmitted the Usool ud-Deen to us by their Ijmaa’ as they have transmitted to us the Qur’aan Al-Kareem and the Sunnah An-Nabawiyah which indicates and guides to the importance and significance of Al-Ijmaa’.
6 – Ahkaam have been established by way of Al-Ijmaa’ which have had a major impact and influence upon the life of the Muslims, the continuation of their State and the preservation of their Deen. Some of these Ahkaam are as follows:
A – The Ijmaa’ As-Sahaabah upon making a copy of the Qur’aan Al-Kareem in the time of the Khalifah ‘Uthmaan Ibn ‘Affaan (ra) and prior to that in the era of the Khalifah Abu Bakr As-Siddeeq (ra) which facilitated and made it easy upon the Muslims to refer to the Masaahif (pl. of Mushaf) to safeguard their memorisation and preservation of the Qur’aan.
B – The Ijmaa’ of the Sahaabah upon Abu Bakr As-Siddeeq (ra) assuming the post of the Khalifah (successor) to the Messenger of Allah (saw) and then after him ‘Umar and ‘Uthmaan (rah). This is what made clear the Shar’iy method for the Muslims, to pledge allegiance to a Khalifah in order to apply Islaam upon them internally and to carry it to mankind externally.
This is as the Sahaabah (rah) left the body of the Messenger of Allah (saw) lying in the house of ‘Aa’ishah (ra) for three days without burying him whilst they occupied themselves in choosing a Khalifah from amongst them and gave him the Bai’ah (pledge). (Translators note: This opinion has been later revised to two nights). This represented an Ijmaa’ indicating that it is Haraam for the Muslims to remain two nights or three days without appointing a Khalifah for them and this is because they delayed the Waajib of burying the Messenger of Allah (saw) due to being sinful as a result of the absence of a Khalifah. (Translators note: This has later been revised to the Ijmaa’ indicating the obligation to engage in the appointment of the Khalifah as soon as the position becomes vacant whilst the obligation to appoint the Khalifah within three days has been deduced from the Ijmaa’ upon the statement of ‘Umar (ra) when he gave a time limit of three days for a Khalifah to be appointed after him).
Hujjiyat ul-Ijmaa’ (the proof of its validity as a Daleel Shar’iy)
The Hujjiyah of Al-Ijmaa’ as a source for the Islamic Ahkaam is a place of agreement amongst the majority of the Fuqahaa and ‘Ulamaa of Usool ul-Fiqh in addition to what is built upon that in terms of the obligation to follow and not violate a ruling based upon it. However, they differed in respect to which Muslims the Ijmaa’ is taken from. The most well-known Ijmaa’s that the ‘Ulamaa of Usool ul-Fiqh and Fuqahaa adopted are:
Ijmaa’ As-Sahaabah, Ijmaa’ Ahl ul-Madinah, Ijmaa’ Al-Ummah Al-Islaamiyah in a certain era or period, Ijmaa’ Al-Mujtahideen in a certain era or time and Ijmaa’ Ahl ul-Bait in the view of the Shee’ah.
I will first deal with the Hujjiyah of Ijmaa’ As-Sahaabah because most of the ‘Ulamaa from the Usooliyeen and Fuqahaa counted it as a legitimate and valid Ijmaa’ and then I will address the Ijmaa’ of the Islamic Ummah.
Hujjiyah Ijmaa’ As-Sahaabah
The Ijmaa’ As-Sahaabah is regarded as a Hujjah (proof) due to the following evidences:
1 - Allah’s commendation and praise (Thanaa’) upon the companions (Sahaabah) of the Messenger of Allah (saw) in the Qur’aan:
Muhammad (saw) is the Messenger of Allah, and those who are with him are severe against the disbelievers and merciful among themselves (Al-Fat’h 29).
And in Soorah Al-Hashr Allah Ta’Aalaa said:
They seek a bounty from Allah and [His] pleasure and supporting Allah and His Messenger, [there is also a share]. Those are the truthful (Al-Hashr 8).
And He Ta’Aalaa said in the following Aayah of Soorah Al-Hashr:
It is those who will be successful (Al-Hashr 9).
And He Ta’Aalaa said:
And the first forerunners [in the faith] among the Muhajireen and the Ansar and those who followed them with good conduct - Allah is pleased with them and they are pleased with Him, and He has prepared for them gardens beneath which rivers flow, wherein they will abide forever. That is the great attainment (At-Taubah 100).
2 – The Sahaabah (rah) are those who gathered together the Qur’aan, they are those who preserved it, transmitted and made copies of it. And Allah Ta’Aalaa says:
Verily it is We Who revealed the Dhikr (i.e. the Qur'an) and verily We will safeguard it (Al-Hijr 9).
Therefore, in the case where it was the Sahaabah (rah) who (practiacally) attended to the Qur’aan through their Ijmaa’ whilst Allah Ta’Aalaa had assumed its preservation, then the Aayah guides to the truthfulness and veracity of their Ijmaa’.
3 – It is impossible in accordance to the Shar’a for the Sahaabah to have an Ijmaa’ upon an error. If that was permissible, then it would be permissible for there to be an error or mistake in respect to the Deen because they are those who have transmitted this Deen to us via their Ijmaa’ (consensus).
4 – The Ijmaa’ As-Sahaabah discloses and is revealing of a Daleel Shar’iy. That is because they did not agree and hold a consensus upon a certain Hukm being a Hukm unless they had a Daleel from the statement, action or Taqreer (approval and consent) of the Messenger of Allah (saw). This reality does not apply to any other than the Sahaabah (rah) because they lived at the same time as the Messenger (saw), gathered around him and transmitted from him.
From these evidences above it becomes evident that the Ijmaa’ As-Sahaabah is the Ijmaa’ which the Adillah (evidences) have guided to being Qat’iy Ath-Thuboot (established and proven decisively). That is because its evidences are from the definite Aayaat and they are Qat’iy Ad-Dalaalah in respect to the commendation and praise upon their truthfulness. This is also the case in respect to the fourth Daleel which is ‘Aqliy (rational).
In addition to the definite Adillah there are also Aahaad Al-Ahaadeeth that have been mentioned to us in respect to their praise like his Qawl (saw):
خَيرُ القُروي ن قَ رْنِ الذي ب عيثْتُ فييهمThe best generation is my generation that I have been sent amongst them
أَصْحابِ كَالنُّجويم يبِييِّيهيم اقْ تَدَيْ تُم اهْتَدَيْ تُمMy companions are like the stars. Any one of them you follow you are guided
(Al-Baihaqi and Ad-Dailamiy related it from Ibn ‘Abbaas).