Three things are binding regardless of whether they were expressed seriously or jokingly: marriage, divorce, and restoration (of a divorcee)
عن أبي هريرة أن رسول الله صلى الله عليه وسلم قال: "ثلاث جِدُّهُنَّ جِدٌّ، وهَزْلُهُنَّ جِدٌّ: النكاح، والطلاق، والرَّجْعَةُ".
Explanation
This Hadīth shows that anyone who pronounces words of marriage, divorce, or restoration of a divorcee, even jokingly, his words are legally binding, for in these rulings, words are binding regardless of whether they were serious or not. For example, if a man conducts a marriage contract of his female authorizer, divorces his wife, or restores her verbally, his words become legally binding as soon as he pronounces them regardless of whether he is serious or not.
In this type of contracts, the option to revoke in the same session or based on specific stipulations is not available. As these three matters are given great importance in the Shariah, they are not liable to playful treatment or joking, and thus mere utterance of their words makes them legally binding.
Source
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