Hadith 3:158
Abu Sa'id al-Khudri reported God’s messenger as saying, “When any of you has intercourse with his wife and wishes to repeat it, he must perform ablution between the two acts.” Muslim transmitted it.
وَعَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِذَا أَتَى أَحَدُكُمْ أَهْلَهُ ثُمَّ أَرَادَ أَنْ يَعُودَ فَلْيَتَوَضَّأْ بَينهمَا وضُوءًا» . رَوَاهُ مُسلم
Original Reference: Mishkat al-Masabih 454