The Messenger of Allah ﷺ was asked about a man who sleeps through a prayer or forgets it. He said, "Its expiation is to pray it as soon as he remembers."
أخبرنا حميد بن مسعدة عن يزيد قال حدثنا حجاج الأحول عن قتادة
سئل رسول الله ﷺ عن الرجل يرقد عن الصلاة أو يغفل عنها قال كفارتها أن يصليها إذا ذكرها
[Sahih] Ibn Majah narrated it in "the Book of Prayer, chapter on one who sleeps through the prayer or forgets it," no. 695, from the hadith of Yazid bin Zuray, with this wording, and it is in "al-Kubra," no. 1585. Muslim also narrated it, see the previous hadith, from the hadith of Qatada, with this wording.