Is it a sin to sleep through Fajir prayer?

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It is not the sleep that is the sin, because the sleeping person is not morally responsible. Rather, the sin would relate to that which directly led one to miss the prayer, such as sleeping too late, or not taking one’s sufficient means to wake up.

Sleeping through Salah should never be intentional as this would risk the possibility of ever being able to make up for it.
Anas reported God’s Messenger as saying, “If anyone forgets a stated prayer or oversleeps, expiation is made by observing it when he remembers it.” In another narration, he said, “That is the only expiation for it.”
Bukhari and Muslim