Mukhtasar Bukhari (Abridged) 202 — Bath becomes obligatory when

Narrated by Sayyiduna Abu Huraira (may Allah be pleased with him) from the Noble Prophet (peace and blessings of Allah be upon him) that he (peace be upon him) said: "When a man sits between the four corners of his wife and then strives with her (for intercourse), then indeed ghusl becomes obligatory."