"When (a man) sits between the four parts of his wife's body and exerts himself, then Ghusl becomes obligatory." (Using translation from Nasāʾī 191)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ «إِذَا جَلَسَ بَيْنَ شُعَبِهَا الْأَرْبَعِ ثُمَّ اجْتَهَدَ فَقَدْ وَجَبَ الْغُسْلُ»