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