The Messenger of Allah ﷺ said, "Whoever forgets a prayer or sleeps through it, its only expiation is to perform it when he remembers it."Yazid said, "Its expiation is that..."
حدثنا إسحاق بن يوسف الأزرق عن ابن أبي عروبة ويزيد بن هارون أخبرنا سعيد عن قتادة
قال رسول الله ﷺ من نسي صلاة أو نام عنها فإنما كفارتها أن يصليها إذا ذكرها قال يزيد فكفارتها أن
Its chain of transmission is sahih. The hadith is found in Bukhari 2/70 no. 597 in the Times of Prayer chapter, section on one who forgets a prayer, and Muslim 1/477 no. 684 in the Mosques chapter, section on making up a missed prayer, and Abu Dawud 1/121 no. 442, and Tirmidhi 1/335 no. 178, section on what has been said about a man who forgets.