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." (RU)
English reference (RU): Vol. 1, Book 1, Hadith 191 (RU)
Arabic reference (RU): Book 1, Hadith 192 (RU)