"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»  

"The Messenger of Allah said: 'Whoever forgets a prayer, let him perform it when he remembers it.'" (Using translation from Ibn Mājah 696)  

قَالَ رَسُولُ اللهِ ﷺ «مَنْ نَسِيَ صَلَاةً فَلْيُصَلِّهَا إِذَا ذَكَرَهَا»