Sunan an-Nasai 15352
In-book no 616أَخْبَرَنَا 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 " .
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.'"