If he sits between her four limbs and exerts himself, the ritual bath becomes obligatory
Authentic hadithعن 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)).
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."
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.
Source · Narrated by Muslim - Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).