It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When a man sits between the four parts of a woman and has intercourse with her, then ghusl becomes obligatory upon him." (That is, if he sits in the position of four limbs and makes an effort for intercourse, then ghusl becomes obligatory.)