What is the expiation for one who missed prayers while asleep and had no one to wake him?

Whoever misses a prayer due to sleep, forgetfulness, or a legitimate excuse, must make it up immediately upon remembering it, even if it is during a forbidden time, because the Prophet, peace and blessings be upon him, said: "Whoever forgets a prayer, let him pray it when he remembers it, for there is no expiation for it except that." He must also strive to take the means that help him wake up for prayer.