He—may Allah Most High have mercy on him—was asked:

“Concerning the ‘brotherhood’ practised by some people nowadays, in which each pledges himself by saying, ‘My wealth is your wealth, my blood is your blood, and my child is your child,’ and the other says the same, and one drinks the other’s blood: is this practice religiously sanctioned or not? If it is not religiously sanctioned and commendable, is it nevertheless permissible? Does it give rise to any of the rulings of Islamic law that apply to actual brothers? What is the meaning of the brotherhood that the Prophet, peace and blessings be upon him, established between the Emigrants and the Helpers?”

He answered:

Praise be to Allah, Lord of all worlds. This practice, in the form described, is not religiously sanctioned, by agreement of the Muslims. The original brotherhood was that established by the Prophet, peace and blessings be upon him, between the Emigrants and the Helpers. He bound them together in an alliance in the house of Anas ibn Mālik. For example, he established brotherhood between Sa‘d ibn al-Rabī‘ and ‘Abd al-Rahmān ibn ‘Awf, so that Sa‘d said to ‘Abd al-Rahmān, “Take half my wealth, and choose either of my two wives so that I may divorce her and you may marry her.” ‘Abd al-Rahmān replied, “May Allah bless your wealth and your family. Show me the way to the market.” Likewise, he established brotherhood between Salmān al-Fārisī and Abū al-Dardā’. All of this is recorded in the Sahīh.