Sunan an-Nasa'i 614
سنن النسائي#614Sahihأَخْبَرَنَا 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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 6:121