Sayyiduna Abu Hurairah (may Allah be pleased with him) reports that the Noble Prophet (peace and blessings be upon him) said: When a man sits between the four parts of a woman and exerts himself for intercourse with her, then ghusl (ritual bath) becomes obligatory.
Hadith Referenceاللؤلؤ والمرجان / كتاب الحيض / 199
Hadith Gradingمحدثین:«صحیح»
Hadith Takhrij«صحیح، أخرجه البخاري في: 5 كتاب الغسل: 28 باب إذا التقى الختانان»
Brief Explanation
Imam Nawawi rahimahullah states that the meaning of the hadith is that ritual bath (ghusl) is not conditional upon the emission of semen (mani), rather, whenever penetration (dukhul) has occurred, ritual bath becomes obligatory upon both parties. And now, there is no difference of opinion regarding this. (Raaz)