Written in Turkish by Prof Dr. İsmail Lütfi ÇAKAN
Translated into English by Furkan YORULMAZ
عَنْ عَبْدِ اللَّهِ بْنَ عُمَرَ رضى الله عنهما قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم : الْمُسْلِمُ أَخُو الْمُسْلِمِ
Allah's Messenger (ﷺ) said, "A Muslim is a brother of another Muslim.”
Like every system, Islam has established its union on obvious essentials. In belief it is based on oneness; in union it is based on brotherhood. Hence, Islam community is a community of brothers which its borders is drawn by Islam belief. To this community and brotherhood, nothing but belief, for instance, neither race, nor colour, nor geography can draw borders. The only precondition of Islam brotherhood is to say “'lā ilāha illā -llāh, muḥammadur rasūlu –llāh.”. Everyone who said this kalimah tawheed is Muslim, and he is a religious fellow of other Muslims. In our Almighty Book Qur’an, this truth is clearly determined as “The believers are but brothers”  and declared. Also, our prophet (ﷺ) said "A Muslim is a brother of another Muslim.”, He (ﷺ) announced that every Muslims are religious brothers of each other no matter where he lived in the world or no matter which age he lived.
In fact, when he (ﷺ) honoured Madinah, He (ﷺ) declared one of muhajirs that came from Mecca as brothers with a Muslim of Madinah, thus he began the first Islam community with brotherhood practice and basics. The organism that modern world looks for and says “social solidarity”, our Prophet, as an example to his ummah who will live until the Day of Judgment, performed the practice of brotherhood in the first Islamic society.
Brotherhood is blessed and a strong relationship. Also, it is a big responsibility, growing with the same amount of the believers of the same religion. About the topic how relationships of brothers should be, in Qur’an also in Hadith, there are a lot of advices and warnings. The word of “brotherhood” explains the essence of these warnings and advices, That is why our Prophet (ﷺ) advised Muslims to take their own self-esteem in every relationship between brothers, “None of you will have faith till he wishes for his (Muslim) brother what he likes for himself.” 
This hadith presents the responsibility of Islam brotherhood with its details. Nevertheless, to make evident this topic, it would be beneficial to point out requirements of religious brotherhood with the help of hadiths.
A Muslim, whether it is based on a valid reason, cannot be offended more than three days. While Muslim brothers stand by him, he cannot make friends with unbelievers or other religion believers. This is because a friend of a Muslim is just a Muslim. As our ancestors said, “A hide cannot be from a pig, and a friend cannot be from an infidel”
A Muslim does not hold a grudge or does not ignore his Muslim brothers. Also, he does not put artificial measures between them. This is because superiority is only with taqwa. Both as an individual or as a nation, he must always use his choices the side of his Muslim brothers. In today’s world, in international platforms the supporting of Muslims each other is important for balance powers in the world. About that topic that hadith is really interesting: “A Muslim is a brother of another Muslim, so he should not oppress him, nor should he hand him over to an oppressor. Whoever fulfilled the needs of his brother, Allah will fulfil his needs; whoever brought his (Muslim) brother out of a discomfort, Allah will bring him out of the discomforts of the Day of Resurrection, and whoever screened a Muslim, Allah will screen him on the Day of Resurrection.” 
As understood from that hadith, the deeds that have been done within the scope of religious brotherhood going to be paid for. That means brothers are “hereafter investment” for each other.
About the relationships between the religious brothers, lots of points are remarked in Hujurat. Reminding these will be appropriate here:
The news that told by a person who sins openly should be searched and shouldn’t be believed instantly. Especially today, correcting the news, specially about Muslims, that came from controlled sources has become a mission.
The Muslim groups that are conflicting each other should be reconciled, and they should be ruled in terms of right and justice. All types of ways should be used to settle the wrong side with the limits set by Allah. And the relationships between brothers should be corrected.
Male Muslims should not make fun of other male Muslims, and female Muslims should not make fun of other female Muslims; and they should not forget that they may be more precious to Allah than them. They should not call them with bad nicknames, or should not have bad suspicion about them, and should not search their secrets, and should not make gossip about them. 
All these indicate that a Muslim should not harm his brothers with his tongue or hand, and should not think bad things about them. Already our prophet (ﷺ) described a mature Muslim that a Muslim who is sure about him and his tongue.
A Muslim should not look down on other Muslim brothers, even he should prefer them instead of himself. He wishes welfare for his religious brothers with his prays. This is because the pray that he performed for his religious brothers is acceptable.
Muslim will know and find his power, happiness, nation, majesty, dignity, honour in his religious brothers.
Muslim will share a Muslim’s happiness, and will share his pain. It is a requirement of “brotherhood” for all Muslims to help their religious brothers who are in trouble or suffered a persecution. It is said in the Qur’an: “And it was said to the people, “Will you congregate” 
Also, our prophet (ﷺ) said: “Help your brother, whether he is an oppressor or he is an oppressed one. People asked, "O Allah's Messenger (ﷺ)! It is all right to help him if he is oppressed, but how should we help him if he is an oppressor?" The Prophet (ﷺ) said, "By preventing him from oppressing others.” 
In another hadith, the rights of the Muslims on the other Muslims are listed as: “When you meet him, offer him greetings; when he invites you to a feast accept it. when he seeks your council give him, and when he sneezes and says: "All praise is due to Allah," you say Yarhamuk Allah (may Allah show mercy to you); and when he falls ill, visit him; and when he dies, follow his bier.” 
Religion brotherhood in Islam as emotion, thought, attitude, sadness, pain and happiness means that all believers are one body and feel this way. Thus, this topic is explained in hadiths as:
“You see the believers as regards their being merciful among themselves and showing love among themselves and being kind, resembling one body, so that, if any part of the body is not well then the whole body shares the sleeplessness (insomnia) and fever with it.” 
“A faithful believer to a faithful believer is like the bricks of a wall, enforcing each other.” 
The ones that who do not have such a warm, sincere and close relations and who do not worry about problems of his religious brothers lost the right to be one of them. They are obliged and even condemned to remain Muslims, to be with Muslims and to behave each other as brothers and sisters. This is because there is no strong relation than religious brotherhood. Thus, Muslim hearts should always beat with love to the Muslims. This is because life is good if it is lived with religious brothers.
Click on the link for the Turkish translation of the article: https://www.siyerinebi.com/tr/prof-dr-ismail-lutfi-cakan/islam-kardesligi
 Buharî, Mezâlim 3; Müslim, Birr 32; Ebû Dâvûd, Edeb 28; Tirmizî, Hudud 3, Birr 18; Ahmed b. Hanbel, Müsned, II, 9, 28; V, 24, 71
 Al-Hujurat (49), 10
 Buhârî, İman 7: Müslim, İman 71, 72; Tirmizi, Kıyame 59; Nesai, İman, 19, 33
 Buhârî, Mezâlim 3; Müslim, Birr 32
 Al-Hujurat (49), 6-12
 Ash-Shu'ara (26), 39
 Buhârî, Mezalim 4
 Müslim, Selâm 4-6
 Buhârî, Edeb 27; Müslim, Birr 66
 Buhârî, Salât 88, Mezâlim 5; Müslim, Birr 65; Tirmizî, Birr 18