Praise be to Allaah.
Ghusl is obligatory on men and woman alike when one of the two following things happens:
When the two “circumcised parts” meet, which is penetration, i.e. intercourse – even if there is no ejaculation – because the Prophet (peace and blessings of Allaah be upon him) said: “When the two circumcised parts meet and the tip of the penis disappears, ghusl becomes obligatory, whether ejaculation takes place or not.” (Narrated by Abu Dawood; see Saheeh Sunan Abi Dawood, 209).
2. Ejaculation – even if the two circumcised parts do not meet – whether this happens as a result of manual stimulation or otherwise. The Prophet (peace and blessings of Allaah be upon him) said: “Water is for water” – i.e., ghusl is required when water (meaning semen or sexual fluid) is emitted.
With regard to fasting, it is not essential for a person to do ghusl before Fajr starts in order for his fast to be valid. If he starts a day of fasting whilst he is junub (impure as a result of sexual activity) his fast is till valid. (See: 70 issues related to fasting). But he should hasten to do ghusl so that he can pray Fajr on time.
And Allaah knows best. May Allaah bless our Prophet Muhammad.
Sheikh Muhammed Salih Al-Munajjid