How can one determine which of the two people who loved each other then separated committed the sin, in accordance with the Hadith: "Two people do not love each other and then separate except due to a sin committed by one of them"?

In Musnad Ahmad, it is narrated from Ibn 'Umar that the Prophet, peace and blessings be upon him, said: "A Muslim is the brother of a Muslim; he neither wrongs him nor abandons him." And he also said: "By Him in whose hand is the soul of Muhammad, two people do not love each other and then are separated except due to a sin committed by one of them." This hadith was graded hasan by Ahmad Shakir and sahih by Shu'ayb al-Arna'ut. The pertinent part of the hadith is also found in Sahih al-Adab al-Mufrad from the narration of Anas.

Al-Sha'bi mentioned: "Two people do not love each other for the sake of Allah and then are separated except due to a sin committed by one of them, and the one who commits the sin is the worse of the two."

The Prophet, peace and blessings be upon him, did not specify the one who committed the sin or the sin itself. The principle is that a servant should think ill of himself and well of his brother, based on his saying, peace and blessings be upon him: "One of you sees the speck in his brother's eye but forgets the log in his own eye."

Al-Manawi stated that separation is a punishment for sin. Musa al-Kazim said: "If your companion changes towards you, know that this is due to a sin you have committed, so repent to Allah from every sin so that his affection for you may be restored." Al-Muzani said: "If you find harshness from your brothers, then repent to Allah, for you have committed a sin. And if you find increased affection from them, that is due to an act of obedience you have performed, so thank Allah the Almighty."