Allah, the Blessed and Exalted, says: "And those among you who make zihar from their wives and then go back on what they said, then [upon them is] the freeing of a slave before they touch one another." (to the end of the verse) [Surah al-Mujadilah: 3]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / Q15255
Imam al-Shafi'i (may Allah have mercy on him) said: What I have retained of what I heard concerning "then go back on what they said" [Surah al-Mujadilah: 3] is that the one who makes zihar has made his wife unlawful through zihar; so when a period of time passes after his pronouncement of zihar in which he has not made her unlawful through the divorce by which she becomes unlawful, nor through anything that would give him a way out of her being made unlawful by it, then the expiation of zihar has become obligatory upon him. It is as if they hold that when he retains what he had made unlawful upon himself, as being lawful, he has gone back on what he said, contradicting it, and has made lawful what he had made unlawful. He said: I do not know any meaning more fitting for it than this. Imam al-Shafi'i (may Allah have mercy on him) said: I know of no one who disagrees that the expiation of zihar is due from him even if he does not go back with another zihar; so it is not permissible to say something which, as far as I know, no one disagrees is not the meaning of the verse.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / 15255
Abu al-Aliyah al-Riyahi said: Khawlah bint Dulayj was married to a man of the Ansar who was ill-tempered, blind and poor. In the days of ignorance, when a man wanted to separate from his wife, he would say to her: You are to me like the back of my mother. She quarrelled with him over something, and he said: You are to me like the back of my mother. He had a child or two. When she heard him say what he said, she picked up her children and set off hurrying to the Messenger of Allah (peace and blessings be upon him). She found him with Aishah, the Mother of the Believers (may Allah be pleased with her), in her house, and Aishah was washing one side of the head of the Messenger of Allah (peace and blessings be upon him). She stood by him and said: O Messenger of Allah, my husband is poor, blind and ill-tempered; I quarrelled with him over something and he said: You are to me like the back of my mother, and he did not intend divorce. The Prophet (peace and blessings be upon him) raised his head and said: "I do not know anything except that you have become unlawful to him." She humbled herself and said: I complain to Allah of what has befallen me and my children. Aishah moved to wash the other side of his head, and she moved with her and said the same, and she said: I have a child or two by him. The Prophet (peace and blessings be upon him) raised his head towards her and said: "I do not know anything except that you have become unlawful to him." She wept and said: I complain to Allah of what has befallen me and my children. The face of the Messenger of Allah (peace and blessings be upon him) changed, and Aishah (may Allah be pleased with her) said: Step back! So she moved aside. The Messenger of Allah (peace and blessings be upon him) remained thus as long as Allah willed; then the revelation ended, and he said: "O Aishah, where is the woman?" She said: Here she is. He said: "Call her." So she called her, and the Prophet (peace and blessings be upon him) said: "Go and bring your husband." She set off hurrying and before long brought him and took him in to the Prophet (peace and blessings be upon him); he was just as she had said: blind, poor and ill-tempered. The Prophet (peace and blessings be upon him) said: "I seek refuge with the All-Hearing, the All-Knowing from Satan the accursed. In the name of Allah, the Most Gracious, the Most Merciful: 'Allah has heard the statement of the woman who disputes with you concerning her husband and complains to Allah' [Surah al-Mujadilah: 1]" to the end of the verse. Then the Prophet (peace and blessings be upon him) said to him: "Can you free a slave?" He said: No. He said: "Then can you fast two consecutive months?" He said to him: By the One who sent you with the truth, if I do not eat once, twice or three times, my sight almost fails. He said: "Then can you feed sixty poor people?" He said: No, unless you help me with it. So the Messenger of Allah (peace and blessings be upon him) called for it, and he expiated his oath.
This is mursal, but it has supporting narrations (shawahid), and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / 15256