It was narrated from Abu Hurairah that:
the Messenger of Allah (ﷺ) said: "There are three matters in which seriousness is serious and joking is serious: marriage, divorce and taking back (one's wife). " (DE)
English reference (DE): Vol. 3, Book 10, Hadith 2039 (DE)
Arabic reference (DE): Book 10, Hadith 2117 (DE)