Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Whoever forgets to pray a prayer or sleeps through it, its expiation is that he prays it whenever he remembers it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3181