It was narrated that Anas said:

"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.'"

أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، عَنْ يَزِيدَ، قَالَ حَدَّثَنَا حَجَّاجٌ الأَحْوَلُ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، قَالَ سُئِلَ رَسُولُ اللَّهِ ﷺ عَنِ الرَّجُلِ يَرْقُدُ عَنِ الصَّلاَةِ أَوْ يَغْفُلُ عَنْهَا قَالَ " كَفَّارَتُهَا أَنْ يُصَلِّيَهَا إِذَا ذَكَرَهَا " .