Sahih Muslim 343
Sahih Muslim · Book 3: The Book of Menstruation · At the beginning of Islam, intercourse did not necessitate ghusl unless semen was emitted, then that was abrogated and ghusl becomes obligatory for intercourse
Abu Sa'id al-Khudri reported:
حَدَّثَنَا هَارُونُ بْنُ سَعِيدٍ الأَيْلِيُّ ، حَدَّثَنَا ابْنُ وَهْبٍ ، أَخْبَرَنِي عَمْرُو بْنُ الْحَارِثِ ، عَنِ ابْنِ شِهَابٍ ، حَدَّثَهُ أَنَّ أَبَا سَلَمَةَ بْنَ عَبْدِ الرَّحْمَنِ حَدَّثَهُ عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ قَالَ " إِنَّمَا الْمَاءُ مِنَ الْمَاءِ " .
The Apostle of Allah (ﷺ) observed: Bathing is obligatory in case of seminal emission.