Hadith: If he sits between her four limbs and exerts himself, the ritual bath becomes obligatory

عن أبي هريرة رضي الله عنه أَنَّ النَّبِيَّ - صلى الله عليه وسلم - قَالَ: ((إِذَا جَلَسَ بين شُعَبِهَا الأَربع، ثم جَهَدَهَا، فَقَد وَجَبَ الغُسْلُ)) . وفي لفظ ((وإن لم يُنْزِل)).
[صحيح] - [الرواية الأولى: متفق عليها. الرواية الثانية: رواها مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If he sits between her four limbs and exerts himself (by having sexual intercourse with her), the ritual bath becomes obligatory." Another version adds: "even if he does not ejaculate."
[Authentic hadith] - [Narrated by Muslim - Narrated by Bukhari & Muslim]

Explanation

If a man sits between a woman's four limbs, i.e. her arms and legs, and he inserts his male organ into her vagina, then it is obligatory on both of them to take a ritual bath in order to remove the major ritual impurity even if no semen is discharged. Penetration on its own necessitates ritual bath.