Hadith: Whoever forgets a prayer should perform it once he remembers it, and there is no expiation for it except this

عن أنس بن مالك عن النبي صلى الله عليه وسلم قال: "من نسي صلاةً فليُصلِّ إذا ذكرها، لا كفارة لها إلا ذلك {وأقم الصلاة لذكري}" [طه: 14].
[صحيح] - [متفق عليه]

Anas ibn Mālik (may Allah be pleased with him) reported that the Prophet (may Allah’s peace and blessings be upon him) said: "Whoever forgets a prayer should perform it once he remembers it, and there is no expiation for it except this."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The Messenger of Allah (may Allah’s peace and blessings be upon him) said: If anyone forgets a prayer until its time has passed, he should pray it once he remembers. This also applies to a sleeping person when he wakes up, as related in another version that says: "Whoever oversleeps a prayer or forgets it should perform it once he remembers it." There is no expiation for it except this; i.e., nothing can cover it except offering the prayer, beyond which no expiation or fine is required. {And establish prayer to remember Me} [Surat Tāha: 14]. In other words, establish prayer as you remember it, for he who remembers prayer remembers Allah, Exalted be He.