It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:
"When (a man) sits between the four parts of his wife's body and exerts himself, then Ghusl becomes obligatory." (IT)
English reference (IT): Vol. 1, Book 1, Hadith 191 (IT)
Arabic reference (IT): Book 1, Hadith 192 (IT)