Mishkat al-Masabih 3284
Abu Huraira reported God’s Messenger as saying, “There are three things which, whether undertaken seriously or in jest, are treated as serious:
marriage, divorce and taking back a wife after a divorce which is not final.” 

Tirmidhi and Abu Dawud transmitted it, Tirmidhi saying this is a hasan gharib tradition.
وَعَنْ أَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: 
" ثَلَاثٌ جَدُّهُنَّ جَدٌّ وَهَزْلُهُنَّ جَدُّ: النِّكَاحُ وَالطَّلَاقُ وَالرِّجْعَةُ ". رَوَاهُ التِّرْمِذِيُّ وَأَبُو دَاوُدَ وَقَالَ التِّرْمِذِيُّ: هَذَا حَدِيثٌ حَسَنٌ غَرِيب