Sahih Muslim 347
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
وَحَدَّثَنَا عَبْدُ الْوَارِثِ بْنُ عَبْدِ الصَّمَدِ ، حَدَّثَنِي أَبِي ، عَنْ جَدِّي ، عَنِ الْحُسَيْنِ ، قَالَ يَحْيَى وَأَخْبَرَنِي أَبُو سَلَمَةَ ، أَنَّ عُرْوَةَ بْنَ الزُّبَيْرِ ، أَخْبَرَهُ أَنَّ أَبَا أَيُّوبَ أَخْبَرَهُ أَنَّهُ، سَمِعَ ذَلِكَ، مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم .
Abu Ayyub reported that he had heard like this from the Messenger of Allah (ﷺ).