When does ritual purity become obligatory for a husband and wife after intercourse at night in Ramadan for their fast to be valid?

It is recommended for a husband and wife, if one has intercourse with the other at night, to perform ghusl (ritual bath) before dawn breaks. However, if dawn breaks while they are in a state of janabah (ritual impurity), they are only required to perform ghusl, and their fast is valid. The proof for this is that the Prophet, peace and blessings be upon him, would be overtaken by dawn while he was in a state of janabah from his wife, then he would perform ghusl and fast, and he would not make up (that day).