"The Messenger of Allah ﷺ was asked about a man who slept and missed the prayer, or forgot it. He said: 'The expiation for that is to pray it when he remembers it.'" (Using translation from Nasāʾī 614)
سُئِلَ رَسُولُ اللهِ ﷺ عَنِ الرَّجُلِ يَرْقُدُ عَنِ الصَّلَاةِ أَوْ يَغْفُلُ عَنْهَا قَالَ «كَفَّارَتُهَا أَنْ يُصَلِّيَهَا إِذَا ذَكَرَهَا»