He who forgets the prayer, or he slept (and it was omitted), its expiation is (only) that he should observe it when he remembers it. (Using translation from Muslim 684c)
عَنِ النَّبِيِّ ﷺ قَالَ «مَنْ نَسِيَ صَلَاةً أَوْ نَامَ عَنْهَا فَكَفَّارَتُهَا أَنْ يُصَلِّيَهَا إِذَا ذَكَرَهَا»