Sayyidah Maymunah bint Sa'd (may Allah be pleased with her) said: I said: O Messenger of Allah, give us a ruling about a man who forgot the prayer until the sun rose or set; what is its expiation? He (peace and blessings be upon him) said: "When he remembers it, let him pray it and pray it well, and let him perform ablution and perform his ablution well; that is his expiation."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21522