Allah - the One free from all imperfections - said:
“So have taqwa (fear and obedience) of Allah and reconcile the differences between yourselves.”
“Indeed the Believers are but brothers. Therefore reconcile the differences between your brothers, and fear Allah so that you may receive mercy.”
So this is a knot which Allah has bound the Believers together with, such that whenever a person is found possessing iman (faith) in Allah, His Angels, His Books, His Messengers and the Last Day – whether in the eastern or western part of the world – then such a person becomes the brother of the Believers. This brotherhood obligates that the Believers should love for that person that which they love for themselves, and hate for that person that which they hate for themselves.
Therefore, the Prophet sallallahu ‘alayhi wa sallam said – whilst ordering the brotherhood of iman (faith):
“Do not envy one another. Do not inflate prices upon one another. Do not hate one another. Do not forsake one another. Do not under-cut one another. But be worshippers of Allah and brothers. The Muslim is the brother of another Muslim; he neither oppresses him, nor humiliates him, nor lies to him, nor holds him in contempt. And piety is right here [and he pointed to his chest three times]. It is enough evil for a person to hold his brother Muslim in contempt. The whole of a Muslim for another Muslim is sacred; his blood, his property and his honour.’
And he sallallahu ‘alayhi wa sallam said:
“The Believer to the Believer is like a solid structure, one part supporting the other.”
And he interlaced his fingers to demonstrate this.
“Thus Allah and His Messenger sallallahu ‘alayhi wa sallam ordered that the rights of the Believers be fulfilled by each other, and that those matters which will bring about harmony, love and togetherness be carried out. All of this is to be done in order to further the rights that they have over one another.
So from such rights is that when fighting occurs between them – thereby causing their hearts to separate and to have hatred and to cut-off from each other – then the Believers should bring about reconciliation between their brothers and do that which will remove the enmity.
Then Allah ordered taqwa in general and made the bestowal of His Mercy the consequence of their having taqwa and their fulfilling the rights of the Believers.
So He said:
“And have taqwa of Allah in order that you may receive mercy.”
Thus, if Allah’s Mercy is attained, then the good of this world and the Hereafter will also be attained.
‘A’ishah radhiallahu ‘anha used to say, that when people abandoned acting upon this ayah (verse):”When two parties of the Believers fight each other, then make reconciliation between them.”
So when the Muslims began fighting each other, it was obligatory to reconcile them, as Allah - the Most High - ordered. However, when they did not do this, then fitnah (trials and tribulations) spread, as did ignorance, and this is what caused the differences to continue.
Allah’s Messenger sallallahu ‘alayhi wa sallamsaid:
“Shall I not inform you of something that is greater in degree than (optional) fasting, charity and prayer?”
They said: Indeed inform us.
So he said:
“It is reconciling the people. For indeed causing corruption between them is the shaver!”
And he sallallahu ‘alayhi wa sallam explained this shaving in another saying:
“The disease of the people before you has overcome you, namely envy and hatred and it is the shaver. I do not say that it shaves off the heir. Rather, it shaves off the religion!”
[(source) Al-Istiqaamah Magazine, Issue No. 3 - Rabi' ul-Awwal 1417H / August 1996CE]