عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ قَالَ: ((إِذَا جَلَسَ بَيْنَ شُعَبِهَا الْأَرْبَعِ وَأَجْهَدَ نَفْسَهُ (وَفِي رِوَايَةٍ: ثُمَّ جَهَدَهَا) فَقَدْ وَجَبَ الْغُسْلُ أَنْزَلَ أَوْ لَمْ يُنْزِلْ))
It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him and his family) said: When a man sits between the four limbs of his wife and then exerts himself (according to one narration, then causes her [the wife] to exert herself), then ghusl becomes obligatory, whether ejaculation occurs or not.