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."
عَنْ رَسُولِ اللَّهِ ﷺ قَالَ إِذَا جَلَسَ الرَّجُلُ بَيْنَ شُعَبِهَا الأَرْبَعِ ثُمَّ جَهَدَهَا فَقَدْ وَجَبَ الْغُسْلُ