What is the ruling on missing Fajr prayer until sunrise due to ignorance of the obligation to perform ghusl immediately after janabah?

Ghusl for janabah is not immediately obligatory; rather, it is permissible to delay it until a time when one can perform ghusl and catch the prayer within its prescribed time. The Prophet, peace and blessings be upon him, would sometimes be overtaken by dawn while in a state of janabah, then he would perform ghusl and fast. If a person in a state of janabah is overcome by sleep and does not wake up until after the prayer's time has expired, there is no sin upon him, because sleeping before the prayer's time enters is permissible. It should be noted that some scholars hold the view that it is obligatory to take measures to ensure one wakes up. As for delaying purification, which leads to the prayer's time expiring, without being due to sleep, then it is negligence and a squandering of the prayer.