عَنْ أُمِّ سَلَمَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُجْنِبُ ثُمَّ يَنَامُ ثُمَّ يَنْتَبِهُ ثُمَّ يَنَامُ. رَوَاهُ أَحْمد

Abu Rafi' said

One day God’s messenger visited his wives, washing after intercourse with each, so I asked, “Messenger of God, why do you not simply wash once at the end?” He replied, “This is purer, better and cleaner. Ahmad and Abu Dawud transmitted it.