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.  

نَبِيُّ اللَّهِ ﷺ مَنْ نَسِيَ صَلاَةً أَوْ نَامَ عَنْهَا فَكَفَّارَتُهَا أَنْ يُصَلِّيَهَا إِذَا ذَكَرَهَا  

When any one of you omits the prayer due to sleep or he forgets it, he should observe it when he remembers it, for Allah has said:" Observe prayer for remembrance of Me."  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا رَقَدَ أَحَدُكُمْ عَنِ الصَّلاَةِ أَوْ غَفَلَ عَنْهَا فَلْيُصَلِّهَا إِذَا ذَكَرَهَا فَإِنَّ اللَّهَ يَقُولُ أَقِمِ الصَّلاَةَ لِذِكْرِي