And he who does not find [a slave] - then a fast for two months consecutively before they touch one another; and he who is unable - then the feeding of sixty poor persons. That is for you to believe [completely] in Allāh and His Messenger; and those are the limits [set by] Allāh. And for the disbelievers is a painful punishment.
Ahmad recorded that Khuwaylah bint Tha`labah said, "By Allah! Allah sent down the beginning of Surat Al-Mujadilah in connection with me and `Aws bin As-Samit. He was my husband and had grown old and difficult. One day, he came to me and I argued with him about something and he said, out of anger, `You are like my mother's back to me.' He went out and sat with some of his people. Then he came back and wanted to have sexual intercourse with me. I said, `No, by the One in Whose Hand is the soul of Khuwaylah! You will not have your way with me after you said what you said, until Allah and His Messenger issue judgement about our case.' He wanted to have his way regardless of my choice and I pushed him away from me; he was an old man.' I next went to one of my neighbors and borrowed a garment from her and went to the Messenger of Allah ﷺ. I told him what happened and kept complaining to him of the ill treatment I received from `Aws. He said,
ذَٰلِكَ لِتُؤْمِنُوا بِاللَّـهِ وَرَسُولِهِ وَتِلْكَ حُدُودُ اللَّـهِ وَلِلْكَافِرِينَ عَذَابٌ أَلِيمٌ (...This is [ laid down ] so that you believe in Allah and His Messenger. And these are the limits set by Allah. And for the disbelievers there is a painful chastisement. 58:4) This part of the verse contains the phrase 'so that you believe'. Belief here implies 'to act upon the sacred laws, ordinances and injunctions or obey the commandments of Allah and His Messenger'. Then the verse says that the laws of expiation are limits set by Allah. It is prohibited to exceed them. It indicates that in matters of marriage, divorce, incestuous comparison and so on, Islam has abolished all traces of pre-Islamic pagan elements of customs and practices, and set down the just and approved code of conduct. Man is required to stick strictly to the parameters of Islam. Those who oppose and deny the Divine limits will be tormented most severely.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 4–5
Islam distinguishes between form and reality. That is why Islam has not recognised the old custom of a man’s calling a woman ‘mother’, although she is not his real mother, and then treating her as such for the purpose of divorcing her. Any such action is absurd, because it cannot change the laws of nature. It is clarified in the Quran that simply by acting in the above-mentioned manner, a man’s wife does not stand divorced. However, it has been made compulsory for the concerned person to pay a penalty and then go back to his wife. After committing an error, if a man pays a penalty in this way, he revives his faith, which he had discarded due to negligence or stupidity.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.