Sunan an-Nasa'i, Book 1, Hadith 192 (In-Book) ⓘ
It was narrated from Abu Hurairah that the Messenger of Allah said: "When (a man) sits between the four parts of his wife's body and exerts himself, then Ghusl becomes obligatory."
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، قَالَ سَمِعْتُ الْحَسَنَ، يُحَدِّثُ عَنْ أَبِي رَافِعٍ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا جَلَسَ بَيْنَ شُعَبِهَا الأَرْبَعِ ثُمَّ اجْتَهَدَ فَقَدْ وَجَبَ الْغُسْلُ " .
also referenced under: Volume 1, Book 1, Hadith 191 (DarusSalam)