Must the husband and wife perform ghusl after intercourse before the dawn adhan, or is it permissible for them to sleep while in a state of janabah until the morning?

It is better for a fasting husband and wife to purify themselves from major ritual impurity (janabah) before dawn, if possible. However, there is no harm if they delay the ghusl (ritual bath) until after dawn, based on the hadith of Aisha and Umm Salamah (may Allah be pleased with them both) that the Prophet (peace be upon him) would be overtaken by dawn while he was in a state of janabah due to his wife, then he would perform ghusl and fast.