Three things are binding regardless of whether they were expressed seriously or jokingly: marriage, divorce, and restoration (of a divorcee).
[Ibn Maajah]
Original Hadith
"""ثلاث جِدُّهُنَّ جِدٌّ، وهَزْلُهُنَّ جِدٌّ: النكاح، والطلاق، والرَّجْعَةُ"".
[حسن.] - [رواه أبو داود والترمذي وابن ماجه."