Whenever the Messenger of Allah ﷺ intended to sleep after having sexual intercourse, he performed ablution as for the prayer before going to sleep. (Using translation from Muslim 305a)
كَانَ رَسُولُ اللهِ ﷺ إِذَا أَرَادَ أَنْ يَنَامَ وَهُوَ جُنُبٌ تَوَضَّأَ وُضُوءَهُ لِلصَّلَاةِ قَبْلَ أَنْ يَنَامَ