Can a Muslim call a non-Muslim ‘Brother’?

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Yes, it would be permissible to refer to or respond to a non-Muslim as a ‘brother’ referring to the common brotherhood of humanity.

According to some scholars, it would also be permissible to greet a non-Muslim with ‘Assalam ‘alaykum’ and respond to their greeting with ‘Wa’alaykum assalam’. The permission is more desirable if one hopes for their Islam. And Allah knows best.

[Sharh Sahih Muslim]

Warmest salams,
[Shaykh] Jamir Meah

“Abu Bakr (May Allah be pleased with him) is from me and I am from him”

Abu Bakr (رضي الله عنه‎)’s immersion in divine love was so great that he sacrificed his life. He found himself only in the presence of the Prophet. At each meeting with the Prophetﷺ he lost more concern for his own affairs. The more he met with him, the more he wanted to be with him.

Abu Bakr(RA)

The Prophetﷺ said one day, “No one has made his property more available to me than has Abu Bakr.”

Abu Bakr (رضي الله عنه‎) replied in tears, “O Messenger! Are not I and my property for you alone?” (1)

The Prophetﷺ ended the exchange with a famous blessing: “Abu Bakr (رضي الله عنه‎) is from me and I am from him. He is my brother in this world and the next.”(2) The Prophet, by these words, indicated their union in the spiritual world and the connection established between their hearts.

References:

1.  Ibn Maja, al-Fada’il al-ashab al-nabi, 11.

2.  al-Tirmidhi, al-Manaqib, 20.