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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.'"
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، عَنْ يَزِيدَ، قَالَ حَدَّثَنَا حَجَّاجٌ الأَحْوَلُ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، قَالَ سُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَنِ الرَّجُلِ يَرْقُدُ عَنِ الصَّلاَةِ أَوْ يَغْفُلُ عَنْهَا قَالَ " كَفَّارَتُهَا أَنْ يُصَلِّيَهَا إِذَا ذَكَرَهَا " .
sunnah.com