It was narrated from Abu Hurairah that:
The Messenger of Allah said: "When a man sits between the four parts (arms and legs of his wife) and has intercourse, then bath is obligatory." (JA)
English reference (JA): Vol. 1, Book 1, Hadith 610 (JA)
Arabic reference (JA): Book 1, Hadith 653 (JA)